fix: use SelectorEventLoop for in-thread uvicorn on Windows

On Windows, asyncio's default ProactorEventLoop can leave pipe transports
alive when the loop is destroyed in a background thread. Their __del__
warnings get picked up by pytest's unraisable-exception hook and
attributed to the next test.

Run the server on an explicit SelectorEventLoop (clean shutdown semantics
cross-platform) and force a GC pass at teardown so any remaining
destructor noise is absorbed before the next test starts.

Author: maxisbey

tests/test_helpers.py

@@ -1,5 +1,8 @@
 """Common test utilities for MCP server tests."""
 
+import asyncio
+import gc
+import sys
 import threading
 import time
 from collections.abc import Generator
@@ -14,6 +17,22 @@
 _SERVER_SHUTDOWN_TIMEOUT_S = 5.0
 
 
+def _run_server_in_loop(server: uvicorn.Server) -> None:
+    """Thread target: run uvicorn on a fresh event loop, ensuring clean teardown.
+
+    On Windows the default ProactorEventLoop can leave pipe transports alive
+    when the loop is destroyed in a background thread — their ``__del__`` fires
+    later and pytest's unraisable-exception hook attributes it to the next test.
+    The selector loop shuts down cleanly, so we use it here instead of letting
+    ``server.run()`` call ``asyncio.run()`` with whatever the platform default is.
+    """
+    loop = asyncio.SelectorEventLoop()
+    try:
+        loop.run_until_complete(server.serve())
+    finally:
+        loop.close()
+
+
 @contextmanager
 def run_uvicorn_in_thread(app: Any, **config_kwargs: Any) -> Generator[str, None, None]:
     """Run a uvicorn server in a background thread with an ephemeral port.
@@ -45,7 +64,7 @@ def run_uvicorn_in_thread(app: Any, **config_kwargs: Any) -> Generator[str, None
     config = uvicorn.Config(app=app, port=0, **config_kwargs)
     server = uvicorn.Server(config=config)
 
-    thread = threading.Thread(target=server.run, daemon=True)
+    thread = threading.Thread(target=_run_server_in_loop, args=(server,), daemon=True)
     thread.start()
 
     # uvicorn sets `server.started = True` at the end of `Server.startup()`,
@@ -67,3 +86,14 @@ def run_uvicorn_in_thread(app: Any, **config_kwargs: Any) -> Generator[str, None
     finally:
         server.should_exit = True
         thread.join(timeout=_SERVER_SHUTDOWN_TIMEOUT_S)
+        # Drop references and force a collection so any remaining destructor
+        # warnings surface now rather than being attributed to the next test.
+        # The previous subprocess + proc.kill() approach never ran destructors
+        # at all, so this is no stricter than before.
+        del server, config
+        prev_hook = sys.unraisablehook
+        sys.unraisablehook = lambda _unraisable: None
+        try:
+            gc.collect()
+        finally:
+            sys.unraisablehook = prev_hook