fix(tests): 3.14 phantom branch arcs, Windows Proactor transport leak

Two more cases where running in-process exposes issues subprocess hid:

1. Python 3.14 coverage: phantom ->exit arcs on nested async with CMs.
   The WITH_EXCEPT_START suppression-check branch (did __aexit__
   suppress?) gets misattributed through the exception table to outer
   CM lines when the inner body yields. Previously the SSE security
   tests never ran on 3.14 (uvicorn thread crashed before server
   started) so this didn't surface. Add # pragma: no branch to the two
   specific CM lines coverage flags.

2. Windows 3.13 lowest-direct: Proactor socket transports from MCP
   client connections don't always close before GC when clients
   disconnect abruptly from the module-scoped server. The transport
   __del__ ResourceWarning is collected by pytest's unraisable hook
   during a later test. Older httpx (lowest-direct) has worse transport
   cleanup than the locked version. Subprocess-based tests hid this —
   resource warnings in a subprocess die with the subprocess. Add the
   same PytestUnraisableExceptionWarning filter that test_sse.py uses.

Author: maxisbey

tests/client/test_http_unicode.py

@@ -120,10 +120,15 @@ async def unicode_session() -> AsyncGenerator[ClientSession, None]:
     async with app.router.lifespan_context(app):
         transport = httpx.ASGITransport(app=app)
         async with httpx.AsyncClient(transport=transport, follow_redirects=True) as http_client:
-            async with streamable_http_client("http://testserver/mcp", http_client=http_client) as (rs, ws):
+            async with streamable_http_client(  # pragma: no branch
+                "http://testserver/mcp", http_client=http_client
+            ) as (rs, ws):
+                # ^ coverage.py on 3.11+ misses phantom ->exit arcs from nested
+                # async with CMs when the innermost body yields (async generator
+                # frame unwinds through __aexit__ chains). On 3.14 the
+                # WITH_EXCEPT_START suppression-check branch is misattributed
+                # through the exception table to the *outer* CM line too.
                 async with ClientSession(rs, ws) as session:  # pragma: no branch
-                    # ^ coverage.py misses the ->exit arc on 3.11+ when yield is
-                    # nested inside multiple async with blocks
                     await session.initialize()
                     yield session
 

tests/server/test_sse_security.py

@@ -140,7 +140,9 @@ async def test_sse_security_disabled():
     settings = TransportSecuritySettings(enable_dns_rebinding_protection=False)
     with run_server_in_thread(make_app(settings), lifespan="off") as url:
         async with httpx.AsyncClient(timeout=5.0) as client:
-            async with client.stream("GET", f"{url}/sse", headers={"Host": "evil.com"}) as response:
+            async with client.stream(  # pragma: no branch
+                "GET", f"{url}/sse", headers={"Host": "evil.com"}
+            ) as response:
                 # Should connect successfully even with invalid host
                 assert response.status_code == 200
 

tests/shared/test_streamable_http.py

@@ -63,6 +63,15 @@
 )
 from tests.test_helpers import run_server_in_thread
 
+# On Windows, the Proactor event loop's socket transports don't always close
+# cleanly before GC when MCP clients disconnect abruptly from the module-scoped
+# server (e.g. test_streamable_http_client_session_termination opens and drops
+# two clients). The transport __del__ emits ResourceWarning which pytest's
+# unraisable collector picks up during a LATER test. These are Windows asyncio
+# internals, not bugs in the client code under test. Previously hidden by
+# subprocess isolation — subprocess resource warnings die with the subprocess.
+pytestmark = pytest.mark.filterwarnings("ignore::pytest.PytestUnraisableExceptionWarning")
+
 # Test constants
 SERVER_NAME = "test_streamable_http_server"
 TEST_SESSION_ID = "test-session-id-12345"