fix(openai): harden realtime reconnect state

[russellmartin-livekit]

Summary

• serialize websocket sends while replaying committed realtime state
• discard stale connection-bound client events and settle pending response/chat/item futures
• close replay-failure sockets and reject updates once the session becomes terminal
• cancel generic xAI say() work tied to the discarded connection

Why

main already replays committed state, but queued client events and acknowledgement futures survive the failed socket. They can otherwise be sent after replay or leave callers waiting indefinitely.

Test plan

• uv run pytest tests/test_realtime/test_openai_realtime_reconnect.py tests/test_realtime/test_openai_realtime_model.py tests/test_realtime/test_xai_realtime_model.py --unit
• uv run ruff format --check on changed Python files
• uv run ruff check on changed Python files