[frostming]

Problem

MessageSender pushed each frame onto a queue drained by a background loop task, and send() awaited a future the loop settled. Once the loop stopped, nothing settled those futures, so send() could wait forever:

• a write/drain error ended the loop, leaving queued sends and any later send pending;
• a send() after close() enqueued behind the stop sentinel;
• cancelling the loop task (e.g. TaskSupervisor.shutdown()) abandoned in-flight and queued sends.

Fix

The sender only needs to serialize writes, so replace the queue + loop with an asyncio.Lock. StreamWriter.write() buffers a whole frame synchronously, so frames can't interleave; the lock serializes drain() in FIFO order. Writes now run in the caller's task, so errors and cancellation surface directly in send(). After a failed write or close(), further sends raise ConnectionError (chained to the original write error).

MessageSender(writer, supervisor) becomes MessageSender(writer); the only caller is Connection.

Follow-ups exposed by sends no longer hanging:

• Connection._run_request: a response that can't be sent because the connection already closed is dropped (debug log) instead of failing the task.
• TaskSupervisor.shutdown(): uses gather(..., return_exceptions=True), so it no longer re-raises errors from tasks that already failed and were reported by the error handlers (this made Connection.close() raise in test_cancellation_cascades_between_sdk_peers).

Behaviour change

close() no longer waits for queued frames to flush. Sends already inside the lock still complete; sends still waiting for the lock raise ConnectionError.

Testing

• New tests/test_message_sender.py: write order, send after close, write failure fails waiting and later sends, cancelled send releases the lock.
• make check: all checks passed.
• uv run python -m pytest: 493 passed, 1 skipped (3 runs), excluding tests/http/test_websocket.py and tests/http/test_ws_cookies.py. 5 tests in those two files fail in my local environment on main as well, unrelated to this change.

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