Cancel Bulk Sync Schema

Requests cancellation of any running executions for a specific schema on a bulk sync. A schema whose only running work is a backfill returns a conflict.

Cancellation is asynchronous. A successful response means the cancellation signal for this schema has been queued; the schema’s in-flight work continues until the signal is processed. Poll GET /api/bulk/syncs/{id}/schemas/{schema_id} and the parent execution via GET /api/bulk/syncs/{id}/status to confirm the schema has reached a terminal state.