fix: guard ability category registration against double-fire _doing_it_wrong notice

[chubes4]

The notice

PHP Notice: Function WP_Ability_Categories_Registry::register was called incorrectly.
Ability category "block-format-bridge" is already registered.
in /var/www/extrachill.com/wp-includes/functions.php on line 6170

Root cause

On the Extra Chill multisite, WordPress core's wp_abilities_api_categories_init action fires more than once per request. Core's WP_Ability_Categories_Registry::register() warns via _doing_it_wrong whenever an already-registered category gets registered again.

bfb_register_ability_category() called wp_register_ability_category() unconditionally inside its wp_abilities_api_categories_init callback, with no idempotency guard — so on the second hook fire it re-registered the block-format-bridge category and tripped the notice.

The fix

Guard the registration with core's idempotency check wp_has_ability_category() (itself guarded with function_exists since it ships with the same core API). A repeat fire is now a clean no-op. The existing function_exists( 'wp_register_ability_category' ) guard is preserved. No change to the slug, label, description, or hook.

This plugin registers a single category (block-format-bridge); that one call is now guarded.