[Bug]: Align Refund Shipping Line Handling with Order Shipping Mapping

Describe the issue

Codeunit 30246 Shpfy Create Sales Doc. Refund currently writes every refunded shipping charge as a G/L Account line posted to Shpfy Shop."Shipping Charges Account" and unconditionally calls TestField on that account.

This diverges from the sales order flow in codeunit Shpfy Process Order (CreateSalesLines, the shipping-charges loop), which:

1. Looks up Shpfy Shipment Method Mapping by Shop Code + shipping title
2. When the mapping's Shipping Charges Type is set, writes the line as that Type/No. (G/L Account, Item, or Charge (Item))
3. Only falls back to Shop."Shipping Charges Account" when no mapping applies
   Result: Shops that represent freight as an Item (or Charge (Item)) on sales orders are forced to use a G/L account when creating refund credit memos, creating inconsistent shipping line shapes between orders and their corresponding refunds.

Expected behavior

When a refund credit memo is created with a shipping charge:

• The system should look up Shpfy Shipment Method Mapping using Shop Code + RefundShippingLine.Title
• If a mapping exists and Shipping Charges Type <> " ", the shipping line should be created as the mapped Type/No. (G/L Account, Item, or Charge (Item))
• If no mapping exists or Shipping Charges Type = " ", the system should fall back to posting as a G/L Account line to Shop."Shipping Charges Account" (current behavior)
• Refund credit memos should produce the same shipping line shape as the originating sales order
• Shipping Agent Code and Shipping Agent Service Code should be propagated from the mapping

Steps to reproduce

When a refund credit memo is created with a shipping charge:

• The system should look up Shpfy Shipment Method Mapping using Shop Code + RefundShippingLine.Title
• If a mapping exists and Shipping Charges Type <> " ", the shipping line should be created as the mapped Type/No. (G/L Account, Item, or Charge (Item))
• If no mapping exists or Shipping Charges Type = " ", the system should fall back to posting as a G/L Account line to Shop."Shipping Charges Account" (current behavior)
• Refund credit memos should produce the same shipping line shape as the originating sales order
• Shipping Agent Code and Shipping Agent Service Code should be propagated from the mapping

Additional context

• Affected codeunit: ShpfyCreateSalesDocRefund.Codeunit.al — specifically CreateSalesLinesFromRefundShippingLines
• Related codeunit: Shpfy Process Order (CreateSalesLines shipping-charges loop) — this is the order-side pattern to mirror
• Table: Shpfy Shipment Method Mapping (table 30131) is keyed on Shop Code + Name, which covers both order and refund lookups
• Risk: Low. New code is gated on the presence of an explicit mapping row; the default path is unchanged. Feature reuses an already-shipped table and the well-tested order-side pattern.
• No schema changes required

I will provide a fix for a bug

• I will provide a fix for a bug