Assign Fulfillment Center

Assigns a specific fulfillment center to a shipment, overriding the automatically selected one. The caller must have access to the target fulfillment center. **Important — asynchronous inventory re-evaluation:** This action does NOT pre-check inventory availability at the target fulfillment center. The assignment succeeds immediately and inventory allocation re-evaluates the shipment asynchronously; if stock is missing at the target FC the shipment may land in an exception state. It also does not revalidate that the target fulfillment center supports the shipment's current shipping service. Both behaviors mirror the merchant dashboard. **Validation:** The action is rejected when the shipment is already picked, being picked, or sorted by the warehouse; cancelled; shipped or completed; a Kitting shipment; an Automated Amazon FBA shipment; or when the target fulfillment center is inactive or not accessible to the caller. **Note on error handling:** This endpoint returns HTTP 200 for both successful and failed operations. Always check the `is_success` field in the response body; when `is_success` is `false`, the `error` object contains the code and message. HTTP 400 is only returned when the request body itself is missing (null); a missing or non-positive `fulfillment_center_id` is returned as HTTP 200 with `is_success: false`.