Fixed: the product swap list no longer errors when Shopify's search leaves out a variant

The customer portal's swap-product list, and the equivalent swap, edit-order, and select-products modals in the Shopify customer-account extension, could return a server error whenever Shopify's own variant search didn't return every variant Subi had asked for, a partial response that crashed the whole request instead of showing the products that were actually available.

That list now handles a partial response correctly: a variant that Shopify's search leaves out is shown as unavailable to swap to, rather than causing the entire page to fail. Every other variant on the same page still loads and works normally, and browsing, searching, paging, and swapping continue to work as before, so a customer sees every option that's still valid instead of a dead-end error page.

The fix also logs which product or variant ids were missing from Shopify's response, so a future occurrence can be traced back to what Shopify actually returned.