fix: correct inventory service and test for product model changes

- Fix inventory_service.py: use product.vendor_sku instead of non-existent product.product_id
- Fix test: create MarketplaceProductTranslation for title (now in translations table)

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/services/inventory_service.py

@@ -425,7 +425,7 @@ class InventoryService:
             return ProductInventorySummary(
                 product_id=product_id,
                 vendor_id=vendor_id,
-                product_sku=product.product_id,
+                product_sku=product.vendor_sku,
                 product_title=product.marketplace_product.get_title() or "",
                 total_quantity=total_qty,
                 total_reserved=total_reserved,