Pause Run

Pause an active workflow run at the next safe checkpoint. For a running run, the current wave of parallel nodes is allowed to finish before the run parks (in-flight work is preserved, not abandoned). For a pending run that has not yet started, it parks immediately. The run transitions to `paused` status once drained; during the drain period, `status` remains `running` with `pause_requested_at` set. The operation is idempotent: pausing an already-paused run returns it unchanged. A paused run can be resumed via `POST /runs/{run_id}/resume` or permanently stopped via `POST /runs/{run_id}/cancel`.