Creates a refund of a bill from a line selection — the client sends only which parent lines, quantities, batch/serial picks and refund tenders; all money, tax and sign derivation is done server-side from the parent bill. Gated by the company's server-side refunds feature flag (rejected with error code REFUND_NOT_AVAILABLE when off) and not deployed in the docs sandbox, so this operation is documented from code. Requires p/r-bill on the parent bill's location and p/c-bill on the register session's location; register_session_id must be an open register session. Apps without an open POS session can instead create a refund with POST /sales/bills (type: refund, is_cloud_settle: true).