Fix add products to cart parent SKU configurable

[mohaelmrabet]

Description (*)

addProductsToCart mutation ignores parent_sku for configurable products, causing the item to be added without the correct variant options (wrong product type instance processes the buy request).

Two root causes are fixed:

1. QuoteConfigurableOptions\SuperAttributeDataProvider
When selected_options contains no configurable UIDs, the provider now falls back to resolving super_attribute data from parent_sku + child sku using ConfigurableProductGraphQl\Model\Options\Collection — mirroring the existing logic in ConfigurableProductGraphQl\SuperAttributeDataProvider.

2. Quote\Model\Cart\AddProductsToCart::addItemToCart
When parent_sku is present, the service now loads the parent configurable product and passes it to Quote::addProduct() instead of the child simple product. This ensures the Configurable type instance processes the buyRequest with the resolved super_attribute data.

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes addProductsToCart graphql mutation ignores parent_sku for configurable products #40598

Manual testing scenarios (*)

Prerequisites: A configurable product (e.g. SKU MH01) with at least one child variant (e.g. SKU MH01-XS-Black).

Scenario 1 — parent_sku without selected_options (main fix)

1. Create an empty cart: createEmptyCart.
2. Run addProductsToCart with:

cartItems: [{
  quantity: 1
  sku: "MH01-XS-Black"
  parent_sku: "MH01"
}]

3. Check the cart — the item must appear as a ConfigurableCartItem with the correct variant options selected.
4. Before the fix: the item was either rejected or added without variant info.
   Scenario 2 — selected_options (regression check)
5. Create an empty cart.
6. Run addProductsToCart with a base64-encoded configurable UID in selected_options (no parent_sku).
7. Check the cart — behaviour must be unchanged.
   Scenario 3 — invalid parent_sku
8. Run addProductsToCart with a parent_sku that does not own the given child sku.
9. Expect a user-friendly error: "The product ... is not a variant of ...".

Questions or comments

• The fallback reads extension attributes (getConfigurableProductLinks) which are loaded only when the product is fetched via ProductRepository. If the product is already in the object cache this is a no-op.
• Unit tests (8 cases) cover: selected_options path, parent_sku fallback, priority rules, invalid variant, product not found, malformed UID.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)

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[mohaelmrabet]

@magento run all tests

[mohaelmrabet]

@magento run Static tests