AI: Prevent `wp_supports_ai` filter from overriding the `WP_AI_SUPPORT` constant.

When `WP_AI_SUPPORT` is explicitly set to `false`, `wp_supports_ai()` now returns early before the filter runs. This ensures the site owner's explicit preference to disable AI cannot be overridden by a plugin via the `wp_supports_ai` filter.

The filter default is now always `true`, since the constant check happens beforehand.

Developed in: https://github.com/WordPress/wordpress-develop/pull/11295

Follow-up to [62067].

Props justlevine, westonruter, gziolo, mindctrl, adamsilverstein, johnjamesjacoby, ahortin, nilambar, ozgursar, audrasjb, jeffpaul.
Fixes #64706.



git-svn-id: https://develop.svn.wordpress.org/trunk@62239 602fd350-edb4-49c9-b593-d223f7449a82