Discount import/export fixes for Buy X Get Y offers

We’ve fixed several issues with discount imports and exports, particularly for Buy X Get Y offers that target specific products or collections.

🐛 Fixes

• Buy X Get Y exports – correctly exports both customer buys and customer gets items for BXGY discounts.
• Product variant exports – properly handles product variants with SKUs in discount rules (format: product-handle|sku).
• Collection targeting – fixes export of discounts that apply to specific collections.
• Import validation – better error handling for invalid discount IDs during import operations.
• Pagination handling – resolves issues where large discount rules with many products weren’t fully exported.