When the poll loop's safety-net timeout fires (poll_count > max_poll_count),
the background subagent task was abandoned without cancellation or cleanup,
leaving a stale entry in _background_tasks indefinitely.
The original code had a comment promising "the cleanup will happen when the
executor completes", but run_task() in executor.py never calls
cleanup_background_task after reaching a terminal state -- the promise was
never implemented.
This change mirrors the asyncio.CancelledError path: signal cooperative
cancellation via request_cancel_background_task and schedule
_deferred_cleanup_subagent_task to remove the entry once the background
thread reaches a terminal state.
Direct cleanup at poll-timeout time would introduce a race: run_task() could
remove the entry while the poll loop is still mid-iteration, causing a
spurious "Task disappeared" error. The deferred approach avoids this by
waiting for terminal state before removal.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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