gh-155941: Close the transport when a plain client_connected_cb raises - #155942

When a plain-function client_connected_cb passed to asyncio.start_server() raises, StreamReaderProtocol.connection_made() now reports the error via the loop exception handler and closes the transport, mirroring what gh-111601 (gh-110894) did for coroutine callbacks.

Previously the exception propagated out of connection_made(), so the server-side transport stayed open forever, leaking the connection and making Server.wait_closed() hang. The error was also reported with the generic callback message instead of "Unhandled exception in client_connected_cb".

The new test fails without the fix and passes with it. The full test_asyncio suite passes, including -R 3:3 refleak runs on test_streams.

cc. Pycon 2026 KR Sprint @hugovk @corona10

• Issue: asyncio: a plain-function client_connected_cb that raises leaves the server transport open #155941