[Issue] Fix: propagate stock validation message in QuantityValidator error info #40569

This issue is automatically created based on existing pull request: #40556: Fix: propagate stock validation message in QuantityValidator error info

Description (*)

• Fix missing error message when QuantityValidator reports a stock error that already has a stockStateResult with hasError = true
• Previously, addErrorInfo() was called without a message, causing silent errors in consumers like the GraphQL cart mutations
• Now propagates $stockResult->getMessage() with a fallback to the default "out of stock" message

Manual testing scenarios (*)

• Add an out-of-stock product to cart via GraphQL and verify the error response contains a descriptive message
• Verify standard storefront "out of stock" behavior is unchanged
• Run existing CatalogInventory unit/integration tests

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)