Cancel Run

Cancel an active workflow run. Immediately marks the run as `cancelled` and stops any further processing. In-flight work at the current node may be abandoned mid-execution. The operation is idempotent: cancelling an already-cancelled run returns the run unchanged without error. Only runs in `pending`, `running`, or `paused` status can be cancelled. Runs that have already reached a terminal state (`completed`, `failed`, `cancelled`) return 409. Unlike `pause`, cancel is permanent — a cancelled run cannot be resumed. Use `pause` if you intend to continue the run later.