[0xFantomMenace]

subscribe() and unsubscribe() send their frames from background tasks. Two problems:

• The tasks were not kept, and asyncio holds tasks only by weak reference.
• disconnect() closed the socket without waiting for them. An unsubscribe() followed by disconnect() could therefore fail against the closed connection and log Task exception was never retrieved.

Changes:

• Background sends are kept until they finish.
• disconnect() flushes them first, waiting up to 2 seconds.
• A send that finds the connection already closed is dropped quietly, since the server forgets a closed connection's subscriptions. Any other failure goes to the on_error handler.

Tested:

• tests/test_websocket_sends.py: three tests. All three fail on master and pass here.
• The full suite passes (504 passed, 17 skipped).
• ruff and mypy findings are unchanged from master.
• Live against wss://stream.0xarchive.io/ws: three subscribe, unsubscribe and disconnect() cycles, with no loop errors and no unsent frames left.

Part of the unreleased 1.13.0 (CHANGELOG "Fixed").