Callback functions for add_menu_page() should enforce credential-checks

All callback functions for `add_menu_page()` (argument `$function`) should enforce credential-checks, as that is not done automatically (see: https://developer.wordpress.org/reference/functions/add_menu_page/#notes).

Start with the add_menu_page() entry point and read the linked WordPress documentation, especially the notes about the $function callback and credential checks. Then inspect the repository's existing PHP_CodeSniffer rules and tests to determine where this check belongs; done means callbacks passed to add_menu_page() are flagged when credential checks are absent.

Written by the indexing model from the issue text.