BTI-971: match refund by amount for partial REST API line items

[vildanbina]

• REST API refunds don't populate $_POST; admin UI does. Read line items from the order's refund instead.
• Match the refund just created by wc_create_refund by amount (1¢ tolerance).
• HPOS doesn't guarantee get_refunds() order — pick max ID when amounts tie.
• Skip refund rows with no _refunded_item_id or zero qty.
• Normalize WC's negative qty/total to positive for the gateway payload.
• Tests: 15 cases in Test_AbstractPaymentGateway_RefundLineItems.