Two fixes for a race where Discord's typing indicator lingers after the
agent finishes:
1. _keep_typing (root cause): after outer stop_typing() clears the task
dict, _keep_typing wakes from its 2s sleep and calls send_typing()
again, recreating an orphaned loop. Add a finally block so _keep_typing
always calls stop_typing() on exit, cleaning up any loop it recreated.
2. _process_message_background (safety net): add stop_typing() after
cancelling the typing task, catching any platform-level persistent
typing tasks that slipped through.
Combines fixes from PR #2945 by catbusconductor (root cause in
_keep_typing) and PR #2832 by subrih (safety net in
_process_message_background).
65dace1b1a