Cancel Bulk Sync Execution
Requests cancellation of a specific bulk sync execution.
Cancellation is asynchronous. A successful response means the cancellation
signal has been queued; the execution continues to run until the signal is
processed. Poll `GET /api/bulk/syncs/{id}/executions/{exec_id}` until the
execution reaches a terminal state (`completed`, `canceled`, or `failed`) to
confirm cancellation has taken effect.
