A second cancel() call on an interrupted run returned False, causing the
cancel and stream_existing_run router endpoints to raise 409 on double-stop.
Fix: return True inside the lock when record.status == RunStatus.interrupted.
This covers both the POST /cancel and POST /join endpoints without any
re-fetch or extra get() call — the idempotency lives at the source.
Also fixes stream_existing_run (the LangGraph SDK stop-button path), which
had the identical cancel() → 409 pattern and was not covered by the
original PR. Both endpoints share the fix automatically.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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