fix(wishlist-graphql): resolve configured_variant for parent_sku+sku input (#40588)

[lbajsarowicz]

Description

addProductsToWishlist accepts both selected_options (encoded option IDs) and
parent_sku + sku to identify a configurable child. Only the
selected_options input ran through SuperAttributeDataProvider, so when a
caller supplied parent_sku + child sku the buy request never carried
super_attribute, the wishlist item never recorded a simple_product option,
and configured_variant resolved to null.

Added ChildSkuDataProvider: when parent_sku is set, sku differs, and no
selected_options were sent, load both products, walk the parent's
configurable attributes, and emit super_attribute mapping each
attribute_id to the child's stored value. Wired into the existing
BuyRequestBuilder provider list in etc/graphql/di.xml.

Fixes #40588

Fixed Issues

• Fixes addProductsToWishlist mutation is not returning configured variant (Selected variant) while adding child product #40588: addProductsToWishlist mutation is not returning configured variant (Selected variant) while adding child product

Manual testing scenarios

mutation {
  addProductsToWishlist(
    wishlistId: "1"
    wishlistItems: [{ parent_sku: "configurable-parent", sku: "configurable-red", quantity: 1 }]
  ) {
    wishlist {
      items_v2(currentPage: 1, pageSize: 5) {
        items {
          __typename
          ... on ConfigurableWishlistItem { configured_variant { sku } }
        }
      }
    }
    user_errors { code message }
  }
}

Before: configured_variant is null. After: configured_variant.sku == "configurable-red".

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All automated tests passed successfully (183 Wishlist unit tests)
• Changes to the codebase comply with technical guidelines

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[lbajsarowicz]

@magento run all tests

[lbajsarowicz]

@magento run all tests