Bug: CBT does not localize images used as “repeated background” in Cover block

Description

When using Create Block Theme with the “Localize images” option enabled, images used as a repeated background in a Cover block are not being localized into the generated theme.

Instead of being copied into the theme’s assets folder and referenced with a theme-relative URL, the Cover block keeps the original upload URL from the media library, for example:

"url": "http://localhost.local/wp-content/uploads/2025/12/sankofa-texture-one.png"

This breaks the expectation that all media will be bundled with the theme and no longer depend on the original site’s uploads directory.

Steps to reproduce

1. Insert a Cover block.
2. Choose an image from the media library.
3. In the Cover block settings, enable “Repeat background” (image set as repeated background).
4. Use Appearance → Create Block Theme to generate a new theme.
5. In the Create Block Theme UI, make sure “Localize images” is checked.
6. Export, install, and activate the generated theme.
7. Inspect the Cover block in the exported theme (e.g., in the pattern/template file or in the Editor).

Environment

WordPress version: 6.9
Create Block Theme version: 2.7.0
PHP version: 5.7
Theme type: Child theme created via Create Block Theme
Site URL: local development (http://localhost.local)

Start in the Create Block Theme export flow with “Localize images” enabled, then inspect how a Cover block’s repeated background is represented in the generated pattern or template. Done means the image is copied into the generated theme’s assets folder and referenced with a theme-relative URL rather than the original uploads URL.