Comments: Notify mentioned users and thread followers for Notes @mentions

[adamsilverstein]

Backports the notification layer from the Gutenberg Notes @mention work (WordPress/gutenberg#79606, still open upstream). Split out of #12503, which now covers only the kses allowance (the merged WordPress/gutenberg#79604 and WordPress/gutenberg#80221).

Trac ticket: #65639. The kses half remains on #65622.

Upstream status: the source Gutenberg PR (#79606) has not merged yet, so this backport should not land until it does.

What

On rest_insert_comment for note comments (alongside the existing post-author notification), wp_notify_note_mentions():

1. Parses mentions out of the saved content via wp_get_note_mentioned_user_ids(): only anchors carrying both the wp-note-mention class and a user-N class token are treated as mentions, so ordinary links cannot be used to address notifications.
2. Notifies each mentioned user plus any existing followers of the thread, with a short email linking back to the post (wp_send_note_notification()).
3. Subscribes the note author and everyone they mention to the thread. Followers are stored as one _wp_note_followers meta row per user on the thread's top-level note - so concurrent replies cannot clobber each other - and the meta is registered for REST (editable by users who can edit_comment the note) so follower management UI can build on it.

Audience rules:

• The post author is excluded - wp_new_comment_via_rest_notify_postauthor() already notifies them of every note.
• The note's own author is never notified about their own note.
• Recipients are limited to users who can edit_comment the note, matching WP_REST_Comments_Controller::check_read_permission() for notes, so emails cannot leak note content to users who cannot see the note in the editor.
• Everything honors the existing wp_notes_notify option.

New filters: wp_note_notification_recipients, wp_note_notification_subject, wp_note_notification_text.

Why

Mentioning a collaborator in a note is only useful if they hear about it: the notification half closes that loop, and the follower model keeps everyone who has participated in a thread informed of later replies without requiring them to be re-mentioned.

This depends conceptually on the kses allowance in #12503: that is what lets the user-N mention class survive sanitization on a real REST write for users without unfiltered_html. The two are otherwise independent - no shared code paths - so they are reviewed and merged separately.

Testing

Unit tests in tests/phpunit/tests/comment/wpNotifyNoteMentions.php cover mention parsing (plain links ignored, IDs deduped), thread-root resolution, the mentioned-user email + auto-subscribe, author self-exclusion, post-author exclusion, capability gating (a mentioned subscriber is not emailed), follower-of-reply notification, the wp_notes_notify off switch, the recipients filter, follower removal, and the REST meta registration.

npm run test:php -- --group notes

Proposed commit message

Comments: Notify mentioned users and thread followers about new notes.

Port the notification layer from the Gutenberg Notes @mention work. On
rest_insert_comment for note comments, parse the user-N mention classes
out of the saved content, email each mentioned user plus any existing
followers of the thread, and subscribe the note author and everyone they
mention. Followers are stored as one _wp_note_followers meta row per user
on the thread's top-level note so concurrent replies cannot clobber each
other, and the meta is registered for REST so follower management UI can
build on it.

The post author is excluded because wp_new_comment_via_rest_notify_postauthor()
already notifies them of every note, the note's own author is never
notified about their own note, and recipients are limited to users who
can edit_comment the note so note content cannot leak. Everything honors
the wp_notes_notify option.

See related Gutenberg pull request: https://github.com/WordPress/gutenberg/pull/79606.

Props mamaduka.
Fixes #65639.

Trac ticket: https://core.trac.wordpress.org/ticket/65639

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