Asyncio tasks created with create_task() but never stored can be
garbage collected mid-execution. Add self._background_tasks set to
hold references, with add_done_callback cleanup. Tracks:
- /background command task
- session-reset memory flush task
- session-resume memory flush task
Cancel all pending tasks in stop().
Update test fixtures that construct GatewayRunner via object.__new__()
to include the new _background_tasks attribute.
Cherry-picked from PR #3167 by memosr. The original PR also deleted
the DM topic auto-skill loading code — that deletion was excluded
from this salvage as it removes a shipped feature (#2598).
Co-authored-by: memosr.eth <96793918+memosr@users.noreply.github.com>
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