Cancel a Maestro run

Immediately cancels a queued or running Maestro run. The run is marked CANCELLED so its workers stop requesting devices and stop retrying, every flow that has not finished yet is closed out as cancelled, any device session the run already acquired is released, and any request still queued on the grid is dropped. Flows that already completed keep their results and reports. Cancelling is idempotent only in the sense that it is safe to call once: a run that has already reached a terminal state (DONE, FAILED or CANCELLED) returns 409.