fix: return JSON response for user group subscription AJAX saves

[GaryJones]

Summary

• Fixes AJAX handler for user group subscriptions to return a JSON response
• Previously, clicking User Groups checkboxes in the Notifications metabox returned an empty response
• Adds comprehensive integration tests for notifications AJAX functionality

Problem

When clicking on User Groups checkboxes in the Notifications metabox to subscribe/unsubscribe user groups from post notifications, the AJAX request would return an empty response. This caused:

• No visual feedback in the UI when toggling user groups
• Changes not persisting until the post was manually saved

The Users AJAX handler correctly returned wp_send_json_success(), but the User Groups path fell through to wp_die() without any response.

Solution

Added wp_send_json_success() response for User Groups AJAX saves, matching the behavior of User subscriptions:

if ( 'following_usergroups[]' === $_POST['ef_notifications_name'] && $groups_enabled ) {
    $this->save_post_following_usergroups( $post, $user_group_ids );

    if ( defined( 'DOING_AJAX' ) && DOING_AJAX ) {
        wp_send_json_success(
            [
                'subscribers_with_no_access' => [],
                'subscribers_with_no_email'  => [],
            ]
        );
    }
}

Test plan

• Integration tests added (NotificationsAjaxTest.php) covering:
  • Users AJAX save returns JSON success
  • User Groups AJAX save returns JSON success
  • AJAX save fails with invalid nonce
  • AJAX save fails without capability
  • AJAX save persists user subscriptions
  • AJAX save persists user group subscriptions
• Manual testing - User Groups checkboxes now show feedback and persist immediately

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