asyncio proactor server leaks the accepted socket and stops accepting when transport creation fails

Bug report

Bug description:

On the proactor event loop (the default on Windows), BaseProactorEventLoop._start_serving()'s loop() callback runs protocol_factory() and transport creation in the same callback that re-arms accept(), guarded only by except OSError / except CancelledError. When protocol_factory() or _make_ssl_transport() / _make_socket_transport() raises for an accepted connection:

• a non-OSError (e.g. a failing protocol_factory) propagates out of loop(): the accepted socket is never closed (it leaks until garbage collection) and accept is not re-armed, so the server stops accepting new connections;
• an OSError is caught as 'Accept failed on a socket', which closes the listening socket, and still leaks the accepted socket.

This is the proactor counterpart of gh-155934 (selector accept) and gh-153133 (create_connection): an accepted socket must be closed when the transport is never created, and a per-connection failure must not tear down the listening socket or the accept loop.

Verified with mock-based tests in test_proactor_events (the same approach used for the selector fix in gh-155934), which run on all platforms.

CPython versions tested on:

CPython main

Operating systems tested on:

Windows (ProactorEventLoop)

Linked PRs

• gh-156366

Beginne mit BaseProactorEventLoop._start_serving() und den auf Mocks basierenden Tests in test_proactor_events. Reproduziere den Fehler bei der Erstellung des Protokolls oder Transports und überprüfe anschließend, dass der akzeptierte Socket geschlossen wird und der lauschende Server weiterhin Verbindungen annimmt. Vergleiche die bestehende Arbeit im verlinkten PR gh-156366, bevor du Änderungen vornimmst.