Tests: Cover KSES attribute value validation.
Add direct coverage for wp_kses_check_attr_val() length, numeric, valueless, allowed-value, and callback constraints. Check boundaries, strict string matching, and callback arguments, and verify that wp_kses() keeps valid attributes and removes invalid ones.
Developed in: https://github.com/WordPress/wordpress-develop/pull/14321
Props huzaifaalmesbah.
See #65819.