fix(runtime): make RunManager.cancel() idempotent for already-interrupted runs (#3055) (#3058)

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>

backend/packages/harness/deerflow/runtime/runs/manager.py

@@ -258,12 +258,17 @@ class RunManager:
             action: "interrupt" keeps checkpoint, "rollback" reverts to pre-run state.
 
         Sets the abort event with the action reason and cancels the asyncio task.
-        Returns ``True`` if the run was in-flight and cancellation was initiated.
+        Returns ``True`` if cancellation was initiated **or** the run was already
+        interrupted (idempotent — a second cancel is a no-op success).
+        Returns ``False`` only when the run is unknown to this worker or has
+        reached a terminal state other than interrupted (completed, failed, etc.).
         """
         async with self._lock:
             record = self._runs.get(run_id)
             if record is None:
                 return False
+            if record.status == RunStatus.interrupted:
+                return True  # idempotent — already cancelled on this worker
             if record.status not in (RunStatus.pending, RunStatus.running):
                 return False
             record.abort_action = action