CC-40714 Accept the unprefixed availableQuantity validation detail

[matweew]

Fix results: spryker-shop/b2b-demo-marketplace#1411

Summary

• shopping-list-items-configurable-product.cy.ts flakes on b2b-demo-marketplace: should not add a configurable product to the shopping list with empty/string availableQuantity fails with AssertionError: expected response error details ["availableQuantity => This value should be of type numeric."] to contain every detail of one of [...].
• Root cause: on shops whose spryker/api-platform validates productConfigurationInstance as its own top-level constraint tree (rather than as a field nested under the request's attributes), Symfony's Collection validator raises the availableQuantity violation with a property path relative to that tree. TranslatingConstraintViolationListNormalizer::extractFieldName() keeps only the last /-delimited JSON-pointer segment, so there's no productConfigurationInstance segment left to carry — producing a flat availableQuantity => ... detail instead of either of the two shapes this test already tolerated (the untyped leaf vs. the typed whole-object failure).
• Adds that third, already-observed shape to acceptedDetails for both the empty and string availableQuantity cases, following the same tolerant pattern the test already uses for the untyped/typed split, and documents the mechanism in a comment.

Confirmed on b2b-demo-marketplace CI (PHP8.5 MariaDB and PHP8.3 Postgres runs both hit the same flat detail): https://github.com/spryker-shop/b2b-demo-marketplace/actions/runs/36433949382 and https://github.com/spryker-shop/b2b-demo-marketplace/actions/runs/36435049549

Jira: https://spryker.atlassian.net/browse/CC-40714

Test plan

• npx tsc --noEmit passes (verified locally)
• npx prettier --check passes (verified locally)
• CI green on a demoshop that currently reproduces the flat detail (b2b-mp)