feat(android): UPnP SOAP envelope + AVTransport client (UPnP slice 5/6)

SoapClient - minimal SOAP 1.1 envelope builder + POST via the shared
app OkHttpClient. Sets the SOAPACTION + Content-Type headers UPnP
expects, parses the action's Response element as a Map<String,
String>, raises SoapFaultException on a <s:Fault> response with the
UPnP errorCode + errorDescription extracted.

AVTransportClient - thin wrapper over SoapClient pinned to the
AVTransport:1 service. Three actions for v1: SetAVTransportURI /
Play / Stop. Pause + Seek deferred until we have hardware in the
loop to verify per-device quirks.

Three MockWebServer-driven unit tests cover the SOAPACTION header
shape, XML escaping of special chars in arg values, and the fault
response -> SoapFaultException path. kxml2 on the test classpath
(Task 4) makes XmlPullParserFactory resolve on the JVM.

android/app/src/main/java/com/fabledsword/minstrel/player/output/upnp/AVTransportClient.kt

@@ -0,0 +1,50 @@
+package com.fabledsword.minstrel.player.output.upnp
+
+import okhttp3.HttpUrl
+
+/**
+ * High-level wrapper for the UPnP AVTransport service. Three calls
+ * for v1: SetAVTransportURI / Play / Stop. Pause + Seek deferred
+ * until we have hardware in the loop to verify each device's quirks
+ * (Sonos and BubbleUPnP accept the standard shape; some smart TVs
+ * reject Pause without DIDL).
+ */
+class AVTransportClient(
+    private val soap: SoapClient,
+    private val controlUrl: HttpUrl,
+) {
+    suspend fun setAVTransportURI(uri: String, metadata: String = "") {
+        soap.call(
+            controlUrl = controlUrl,
+            serviceType = SERVICE_TYPE,
+            action = "SetAVTransportURI",
+            args = mapOf(
+                "InstanceID" to "0",
+                "CurrentURI" to uri,
+                "CurrentURIMetaData" to metadata,
+            ),
+        )
+    }
+
+    suspend fun play() {
+        soap.call(
+            controlUrl = controlUrl,
+            serviceType = SERVICE_TYPE,
+            action = "Play",
+            args = mapOf("InstanceID" to "0", "Speed" to "1"),
+        )
+    }
+
+    suspend fun stop() {
+        soap.call(
+            controlUrl = controlUrl,
+            serviceType = SERVICE_TYPE,
+            action = "Stop",
+            args = mapOf("InstanceID" to "0"),
+        )
+    }
+
+    private companion object {
+        const val SERVICE_TYPE = "urn:schemas-upnp-org:service:AVTransport:1"
+    }
+}

android/app/src/main/java/com/fabledsword/minstrel/player/output/upnp/SoapClient.kt

@@ -0,0 +1,140 @@
+package com.fabledsword.minstrel.player.output.upnp
+
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+import okhttp3.HttpUrl
+import okhttp3.OkHttpClient
+import okhttp3.Request
+import okhttp3.RequestBody.Companion.toRequestBody
+import org.xmlpull.v1.XmlPullParser
+import org.xmlpull.v1.XmlPullParserFactory
+
+/**
+ * Minimal SOAP/UPnP envelope builder + POST. Hand-rolled rather than
+ * pulled in via jupnp; we control every line and integrate cleanly
+ * with the app's OkHttpClient (shared connection pool, timeouts).
+ *
+ * Builds the standard SOAP 1.1 envelope, POSTs with the required
+ * SOAPACTION + Content-Type headers, returns the parsed
+ * `<action>Response` element as a Map<String, String> (UPnP responses
+ * are flat string maps).
+ *
+ * Throws [SoapFaultException] on a `<s:Fault>` response (the UPnP
+ * device's way of saying "I rejected your request"). Other transport
+ * errors propagate as IOException.
+ */
+class SoapClient(
+    private val okHttp: OkHttpClient,
+) {
+    suspend fun call(
+        controlUrl: HttpUrl,
+        serviceType: String,
+        action: String,
+        args: Map<String, String> = emptyMap(),
+    ): Map<String, String> = withContext(Dispatchers.IO) {
+        val envelope = buildEnvelope(serviceType, action, args)
+        val request = Request.Builder()
+            .url(controlUrl)
+            .post(envelope.toRequestBody(null))
+            .header("Content-Type", SOAP_CONTENT_TYPE)
+            .header("SOAPACTION", "\"$serviceType#$action\"")
+            .build()
+        okHttp.newCall(request).execute().use { response ->
+            val body = response.body?.string().orEmpty()
+            if (!response.isSuccessful) {
+                throw SoapFaultException(faultCodeOf(body), faultDescriptionOf(body))
+            }
+            parseResponseArgs(body, action)
+        }
+    }
+
+    private fun buildEnvelope(
+        serviceType: String,
+        action: String,
+        args: Map<String, String>,
+    ): String = buildString {
+        append("<?xml version=\"1.0\"?>")
+        append("<s:Envelope xmlns:s=\"http://schemas.xmlsoap.org/soap/envelope/\"")
+        append(" s:encodingStyle=\"http://schemas.xmlsoap.org/soap/encoding/\">")
+        append("<s:Body>")
+        append("<u:").append(action)
+        append(" xmlns:u=\"").append(serviceType).append("\">")
+        args.forEach { (k, v) ->
+            append("<").append(k).append(">")
+            append(xmlEscape(v))
+            append("</").append(k).append(">")
+        }
+        append("</u:").append(action).append(">")
+        append("</s:Body>")
+        append("</s:Envelope>")
+    }
+
+    private fun xmlEscape(v: String): String = v
+        .replace("&", "&amp;")
+        .replace("<", "&lt;")
+        .replace(">", "&gt;")
+        .replace("\"", "&quot;")
+        .replace("'", "&apos;")
+
+    private fun parseResponseArgs(body: String, action: String): Map<String, String> {
+        val parser = XmlPullParserFactory.newInstance().newPullParser().apply {
+            setInput(body.reader())
+        }
+        val responseTag = "${action}Response"
+        val args = mutableMapOf<String, String>()
+        var inResponse = false
+        while (parser.eventType != XmlPullParser.END_DOCUMENT) {
+            inResponse = handleParserEvent(parser, responseTag, inResponse, args)
+            parser.next()
+        }
+        return args
+    }
+
+    private fun handleParserEvent(
+        parser: XmlPullParser,
+        responseTag: String,
+        inResponse: Boolean,
+        args: MutableMap<String, String>,
+    ): Boolean = when (parser.eventType) {
+        XmlPullParser.START_TAG -> {
+            if (parser.name == responseTag) {
+                true
+            } else {
+                if (inResponse) {
+                    val name = parser.name
+                    val text = runCatching { parser.nextText() }.getOrDefault("")
+                    args[name] = text
+                }
+                inResponse
+            }
+        }
+        XmlPullParser.END_TAG -> if (parser.name == responseTag) false else inResponse
+        else -> inResponse
+    }
+
+    private fun faultCodeOf(body: String): String =
+        extractBetween(body, "<errorCode>", "</errorCode>") ?: "unknown"
+
+    private fun faultDescriptionOf(body: String): String =
+        extractBetween(body, "<errorDescription>", "</errorDescription>").orEmpty()
+
+    private fun extractBetween(body: String, open: String, close: String): String? {
+        val start = body.indexOf(open)
+        if (start < 0) return null
+        val contentStart = start + open.length
+        val end = body.indexOf(close, contentStart)
+        return if (end < 0) null else body.substring(contentStart, end)
+    }
+
+    private companion object {
+        const val SOAP_CONTENT_TYPE = "text/xml; charset=\"utf-8\""
+    }
+}
+
+/**
+ * Thrown when a UPnP device responds with `<s:Fault>` — typically wraps
+ * a UPnPError with `errorCode` + `errorDescription`. Code is preserved
+ * as a string (UPnP codes are numeric in spec but we don't constrain).
+ */
+class SoapFaultException(val code: String, val description: String) :
+    Exception("SOAP fault $code: $description")

android/app/src/test/java/com/fabledsword/minstrel/player/output/upnp/SoapClientTest.kt

@@ -0,0 +1,146 @@
+package com.fabledsword.minstrel.player.output.upnp
+
+import kotlinx.coroutines.test.runTest
+import okhttp3.OkHttpClient
+import okhttp3.mockwebserver.MockResponse
+import okhttp3.mockwebserver.MockWebServer
+import org.junit.jupiter.api.AfterEach
+import org.junit.jupiter.api.BeforeEach
+import org.junit.jupiter.api.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
+
+/**
+ * Unit tests for SoapClient. Uses MockWebServer to capture the
+ * outgoing SOAP envelope + verify the SOAPACTION header shape, and
+ * to drive the `<s:Fault>` response path.
+ *
+ * kxml2 is on the test classpath (Task 4), so
+ * `XmlPullParserFactory.newInstance()` resolves on the JVM.
+ */
+class SoapClientTest {
+    private lateinit var server: MockWebServer
+    private lateinit var client: SoapClient
+
+    @BeforeEach
+    fun setup() {
+        server = MockWebServer().apply { start() }
+        client = SoapClient(OkHttpClient())
+    }
+
+    @AfterEach
+    fun teardown() {
+        server.shutdown()
+    }
+
+    @Test
+    fun `SetAVTransportURI envelope sent with correct SOAPACTION header`() = runTest {
+        server.enqueue(
+            MockResponse()
+                .setBody(setUriResponseBody())
+                .setHeader("Content-Type", "text/xml"),
+        )
+
+        client.call(
+            controlUrl = server.url("/avt/Control"),
+            serviceType = "urn:schemas-upnp-org:service:AVTransport:1",
+            action = "SetAVTransportURI",
+            args = mapOf(
+                "InstanceID" to "0",
+                "CurrentURI" to "http://server/api/tracks/x/stream?token=t&exp=1",
+                "CurrentURIMetaData" to "",
+            ),
+        )
+
+        val recorded = server.takeRequest()
+        assertEquals(
+            "\"urn:schemas-upnp-org:service:AVTransport:1#SetAVTransportURI\"",
+            recorded.getHeader("SOAPACTION"),
+        )
+        val body = recorded.body.readUtf8()
+        assertTrue(
+            body.contains("<u:SetAVTransportURI"),
+            "envelope missing <u:SetAVTransportURI: $body",
+        )
+        assertTrue(
+            body.contains(
+                "<CurrentURI>http://server/api/tracks/x/stream?token=t&amp;exp=1</CurrentURI>",
+            ),
+            "envelope missing escaped CurrentURI: $body",
+        )
+    }
+
+    @Test
+    fun `XML special chars in args are escaped`() = runTest {
+        server.enqueue(
+            MockResponse()
+                .setBody(setUriResponseBody())
+                .setHeader("Content-Type", "text/xml"),
+        )
+
+        client.call(
+            controlUrl = server.url("/avt/Control"),
+            serviceType = "urn:schemas-upnp-org:service:AVTransport:1",
+            action = "SetAVTransportURI",
+            args = mapOf("CurrentURI" to "a&b<c>\"d'e"),
+        )
+
+        val recorded = server.takeRequest()
+        val body = recorded.body.readUtf8()
+        assertTrue(
+            body.contains("a&amp;b&lt;c&gt;&quot;d&apos;e"),
+            "expected all five XML escapes in body, got: $body",
+        )
+    }
+
+    @Test
+    fun `SOAP fault becomes SoapFaultException`() = runTest {
+        server.enqueue(
+            MockResponse()
+                .setResponseCode(500)
+                .setBody(faultResponseBody())
+                .setHeader("Content-Type", "text/xml"),
+        )
+
+        val ex = assertFailsWith<SoapFaultException> {
+            client.call(
+                controlUrl = server.url("/avt/Control"),
+                serviceType = "urn:schemas-upnp-org:service:AVTransport:1",
+                action = "Play",
+                args = mapOf("InstanceID" to "0", "Speed" to "1"),
+            )
+        }
+        assertEquals("402", ex.code)
+    }
+
+    private fun setUriResponseBody(): String = """
+        <?xml version="1.0"?>
+        <s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"
+                    s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
+          <s:Body>
+            <u:SetAVTransportURIResponse
+                xmlns:u="urn:schemas-upnp-org:service:AVTransport:1"/>
+          </s:Body>
+        </s:Envelope>
+    """.trimIndent()
+
+    private fun faultResponseBody(): String = """
+        <?xml version="1.0"?>
+        <s:Envelope xmlns:s="http://schemas.xmlsoap.org/soap/envelope/"
+                    s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
+          <s:Body>
+            <s:Fault>
+              <faultcode>s:Client</faultcode>
+              <faultstring>UPnPError</faultstring>
+              <detail>
+                <UPnPError xmlns="urn:schemas-upnp-org:control-1-0">
+                  <errorCode>402</errorCode>
+                  <errorDescription>Invalid Args</errorDescription>
+                </UPnPError>
+              </detail>
+            </s:Fault>
+          </s:Body>
+        </s:Envelope>
+    """.trimIndent()
+}