Request a new return for an order.

Required fields:

• order_id
• return_line_items

Unlike the standard create-return endpoint, this endpoint always returns
HTTP 200. Provider-level errors (validation failures, order not found,
bad gateway, etc.) are surfaced via error_code/error_message fields
in the response body instead of HTTP 4xx/5xx status codes. Check
error_code to determine the outcome.