This updates `wp_get_loading_optimization_attributes()` and `wp_maybe_add_fetchpriority_high_attr()` to account for cases where an `IMG` has `fetchpriority=low` or `fetchpriority=auto`:
* `IMG` tags with `fetchpriority=low` are not lazy-loaded since they may be in a Navigation overlay, Details block, or Accordion Item block and need to be loaded the instant the user toggles the block.
* `IMG` tags with `fetchpriority=auto` do not increase the media count since they may be hidden in a viewport by block visibility settings.
* Blocks with conditional visibility (such as hidden on mobile or desktop) now automatically add `fetchpriority="auto"` to their contained `IMG` tags to prevent them from erroneously receiving `fetchpriority=high` or affecting the lazy-loading of subsequent images.
* An `IMG` with `fetchpriority=auto` which also surpasses the `wp_min_priority_img_pixels` threshold will prevent a subsequent image from getting `fetchpriority=high`.
Developed in https://github.com/WordPress/wordpress-develop/pull/11196
Includes backport of [https://github.com/WordPress/gutenberg/pull/76302 Gutenberg#76302].
See related Gutenberg issues:
- [https://github.com/WordPress/gutenberg/issues/76181 76181]: Image in navigation overlay can get `fetchpriority=high` and degrade LCP metric for page.
- [https://github.com/WordPress/gutenberg/issues/76268 76268]: Image in collapsed Details block may erroneously get `fetchpriority=high` even though hidden.
- [https://github.com/WordPress/gutenberg/issues/76301 76301]: Block Visibility: `IMG` in viewport-conditional block may get `fetchpriority=high` even when not displayed.
- [https://github.com/WordPress/gutenberg/issues/76335 76335]: Image in collapsed Accordion block may erroneously get `fetchpriority=high` even though hidden.
Follow-up to r56347, r56037.
Props westonruter, mukesh27, ramonopoly, wildworks.
See #58235.
Fixes#64823.
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