Use parent URL key in configurable variant URL fallback

[juaniten]

Fixes doofinder/support#5082

Summary

• When a configurable variant (visibility=1) has no canonical URL rewrite for its parent, the fallback route was built with the parent's ID but the variant's url_key as the slug, producing a mismatched URL
• Fix injects Magento\Catalog\Model\ResourceModel\Product and uses getAttributeRawValue() to read the parent's url_key directly, so the fallback slug is always consistent with the parent product

[davidmolinacano]

LGTM! 👏