feat(monitors): a DNS monitor can name a resolver; the host page puts Ansible above Docker (#5214)

DNS monitors take an optional Resolver, a server's IP. Set, the check asks
that server directly for A and AAAA over UDP and records how long it took
to answer; empty keeps today's lookup through the container's resolver.
"Does my local DNS still answer for public names" and "does 1.1.1.1" are
now separate monitors. No new dependency.

On a host's page the Ansible card moves above the Docker list, which can
run to dozens of containers and hid it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/core/test_monitors.py

@@ -78,14 +78,16 @@ def test_run_monitor_http_passes_config(monkeypatch):
 def test_run_monitor_dns_passes_expected_ip(monkeypatch):
     seen = {}
 
-    async def fake_dns(address, expected_ip=None):
-        seen["address"], seen["expected_ip"] = address, expected_ip
-        return {"is_up": True, "resolved_ip": "192.0.2.1"}
+    async def fake_dns(address, expected_ip=None, resolver=None):
+        seen["address"], seen["expected_ip"], seen["resolver"] = address, expected_ip, resolver
+        return {"is_up": True, "resolved_ip": "192.0.2.1", "response_ms": 12.5}
 
     monkeypatch.setattr(runner_mod, "dns_check", fake_dns)
     res = asyncio.run(runner_mod.run_monitor(_monitor("dns", "host.test", {"expected_ip": "192.0.2.1"})))
-    assert seen == {"address": "host.test", "expected_ip": "192.0.2.1"}
+    assert seen == {"address": "host.test", "expected_ip": "192.0.2.1", "resolver": None}
     assert res["resolved_ip"] == "192.0.2.1"
+    asyncio.run(runner_mod.run_monitor(_monitor("dns", "host.test", {"resolver": "1.1.1.1"})))
+    assert seen["resolver"] == "1.1.1.1"
 
 
 # ── config validation (shared by the form and the MCP tools) ─────────────────
@@ -107,8 +109,9 @@ def _from_form(form, mtype):
 def test_build_config_from_the_form_per_type():
     assert _from_form(_Form(port="8080"), "tcp") == {"port": 8080}
     assert _from_form(_Form(), "tcp") == {"port": 80}
-    assert _from_form(_Form(expected_ip="10.0.0.1"), "dns") == {"expected_ip": "10.0.0.1"}
-    assert _from_form(_Form(), "dns") == {"expected_ip": None}
+    assert _from_form(_Form(expected_ip="10.0.0.1"), "dns") == {"expected_ip": "10.0.0.1", "resolver": None}
+    assert _from_form(_Form(resolver=" 1.1.1.1 "), "dns") == {"expected_ip": None, "resolver": "1.1.1.1"}
+    assert _from_form(_Form(), "dns") == {"expected_ip": None, "resolver": None}
     assert _from_form(_Form(), "icmp") == {}
 
     http = _from_form(_Form(method="get", expected_status="201", follow_redirects="on"), "http")
@@ -128,6 +131,7 @@ def test_build_config_from_typed_values_defaults_to_safe_checks():
     ("tcp", {"port": "http"}, "port must be a whole number"),
     ("tcp", {"expected_ip": "1.2.3.4"}, "does not take expected_ip; it takes port"),
     ("icmp", {"port": 22}, "takes no config"),
+    ("dns", {"resolver": "dns.google"}, "resolver must be a DNS server's IP address"),
     ("http", {"method": "DELETE"}, "not one of GET, HEAD, POST"),
     ("http", {"expected_status": 42}, "between 100 and 599"),
     ("http", {"timeout_seconds": 0}, "between 1 and 60"),