Fix #40143 - Only X left in stock for configurable product

[thomas-kl1]

Description (*)

Fix the original desired behavior and fetch the first variant of a configurable product.

Manual testing scenarios (*)

• Disable MSI modules and inventory graphql module.
• Query a configurable product with only_x_left_in_stock

Expected: only_x_left_in_stock is the value of its first variant

Result: Error

Questions or comments

Not sure that's the best way to fix it, IMO it should be done on MSI side.

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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[thomas-kl1]

The fix here is still relevant.

[engcom-Hotel]

@magento run all tests

[engcom-Hotel]

Hello @thomas-kl1,

Thanks for the contribution!

The failed tests doesn't related to this PR, but I guess we can add some automated test for the below scenarios:

• Configurable with No Variants - Test null coalescing operator
• Simple Product Regression - Ensure unchanged behavior
• Basic Stock Threshold Logic - Core functionality works