wp_get_registered_image_subsizes() has no unit test coverage

[irozum]

wp_get_registered_image_subsizes() in wp-includes/media.php (since 5.3.0) has no PHPUnit coverage at all — confirmed via grep -rn "wp_get_registered_image_subsizes" tests/phpunit/. It's a pure function composing get_intermediate_image_sizes() and wp_get_additional_image_sizes() into a normalized {width, height, crop} array per registered size, so it's straightforward to cover: custom sizes via add_image_size(), crop as a bool vs. a position array, sizes with no dimensions being omitted, and the intermediate_image_sizes filter. Have a PR ready with 4 tests covering these cases.

wp_get_registered_image_subsizes() (wp-includes/media.php, since 5.3.0) had no PHPUnit coverage at all. It's a pure function that composes get_intermediate_image_sizes() and wp_get_additional_image_sizes() into a normalized {width, height, crop} array per registered image size, so it's straightforward to pin down. This adds 4 tests to Tests_Media:

• A custom size registered via add_image_size() shows up with the correct width/height and crop as false.
• A size registered with a crop position (e.g. array( 'center', 'top' )) preserves that array rather than coercing it to a bool.
• A size with no width/height set is omitted from the result entirely.
• The intermediate_image_sizes filter can exclude a size from the result.

This is a test-only change — no production behavior is added or modified. Each test was verified to actually constrain the code: I temporarily broke the corresponding logic in wp_get_registered_image_subsizes() (inverting the crop bool cast, disabling the zero-dimension skip, disabling the filter application) and confirmed the relevant test went red, then restored the original and confirmed green again.

Ran the full Tests_Media class (453 tests) after adding these — no regressions. Lint and PHPStan clean.

## Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Identifying the coverage gap, implementation, and mutation-testing verification of each assertion. Reviewed by @irozum.