test(mcp): include Host header in raw-ASGI test scope

FastMCP's transport_security module enforces a Host header as DNS-
rebinding protection. Raw-ASGI scope construction doesn't fill it in
automatically (real HTTP clients always send one), so the test
request was getting 421 Misdirected Request with a log warning:

  Missing Host header in request

Production is unaffected — real curl, Claude Code, and any real
client send a Host header.

tests/test_mcp_endpoint.py

@@ -113,6 +113,10 @@ async def test_mcp_endpoint_valid_token_passes_auth():
             status, _ = await _send_request(
                 app, "POST", "/mcp",
                 headers={
+                    # FastMCP's transport_security module enforces a Host
+                    # header (DNS-rebinding protection); without it the
+                    # request gets a 421 Misdirected Request.
+                    "Host": "testserver",
                     "Authorization": "Bearer fmcp_valid",
                     "Content-Type": "application/json",
                     "Accept": "application/json, text/event-stream",