magento/magento2#31353: Materialize GraphQL product image cache with watermarks

[swnsma]

Description

GraphQL product image resolvers returned watermark-aware cache URLs without generating the corresponding cache files. Theme-based catalog:images:resize only builds view.xml sizes, so GraphQL variants (null width/height + watermark params) often 404 or resolve to placeholders.

This change introduces ProductImageCache, used by the media gallery URL resolver, to materialize missing HASH-format product image cache files (including watermarks) during GraphQL URL resolution.

Fixed Issues

1. Fixes [GraphQl] Watermark images don't show. #31353

Manual testing scenarios

1. Configure product image watermarks (Base / Small / Thumbnail) under Content → Design → Configuration.
2. Ensure a product has base, small, and thumbnail images assigned.
3. Clear pub/media/catalog/product/cache (optional but recommended to force generation).
4. Run a GraphQL products query requesting image, small_image, and thumbnail URLs.
5. Open each returned URL in a browser.
6. Expected: Images load successfully and show the configured watermark (not placeholders).
7. Especially verify small_image / thumbnail, which previously failed most often.

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/api-functional tests (where applicable)
• All automated tests passed successfully (all builds are green)

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[swnsma]

@magento run all tests

[swnsma]

@magento run all tests