EST API: Preserve custom status for ability validation errors

he REST run controller for the Abilities API already preserves a custom
HTTP status returned by `wp_ability_normalize_input` filter errors. Apply
the same behavior to validation errors from `wp_ability_validate_input`:
a `WP_Error` is only defaulted to a 400 status when it does not already
include one.

The shared defaulting logic is extracted into a new private
`ensure_error_status()` helper and reused for both normalization and
validation errors.

Follow-up to [62398].
See #64311.



git-svn-id: https://develop.svn.wordpress.org/trunk@62399 602fd350-edb4-49c9-b593-d223f7449a82