Fix 'Show more comments' button in comments metabox not working for custom comment types

[Aljullu]

Right now, the comments metabox in WordPress assumes all comments <tr> elements will have the comment class. But that's not true if the comment type is custom, like WooCommerce's reviews.

This PR makes the selector match the <tr> elements by their id starting with comment-, this seems to be a safer approach because it matches regular comments and other comment types like reviews.

It also changes the default number of reviews which are requested in the AJAX call. Until now, it was 20, but the backend was falling back to 10, causing an inconsistency that made the frontend think all comments were already loaded even if they weren't.

Trac ticket: https://core.trac.wordpress.org/ticket/65392.

Initial report in WooCommerce repo: woocommerce/woocommerce#52731.

Also kudos to @PavloBorysenko for the initial research in woocommerce/woocommerce#52731 (comment).

Use of AI Tools

AI assistance: Yes
Tool(s): Cursor
Model(s): Composer
Used for: Researching the code, brainstorming solutions and writing steps to reproduce.

Steps to reproduce (custom code)

1. Add this code snippet somewhere in your page. tl;dr, it creates a custom post type and makes its comments have the cpt_response type:

add_action( 'init', function () {
        register_post_type(
                'my_cpt',
                array(
                        'labels'       => array(
                                'name'          => 'My custom CPT entries',
                                'singular_name' => 'My custom CPT entry',
                                'add_new'       => 'Add entry',
                                'add_new_item'  => 'Add new entry',
                        ),
                        'public'       => true,
                        'show_ui'      => true,
                        'show_in_menu' => true,
                        'show_in_rest' => true,
                        'supports'     => array( 'title', 'comments' ),
                        'menu_icon'    => 'dashicons-testimonial',
                        'has_archive'  => false,
                )
        );
}, 0 );

add_filter( 'preprocess_comment', function ( $data ) {
        $post_id = isset( $data['comment_post_ID'] ) ? (int) $data['comment_post_ID'] : 0;
        if ( $post_id && 'my_cpt' === get_post_type( $post_id ) ) {
                $data['comment_type'] = 'cpt_response';
        }
        return $data;
}, 1 );

2. In wp-admin, go to My custom CPT entries > Add new entry.
3. Add a title and publish.
4. In the frontend, add at least 21 comments to that entry.
5. Back in the editor, verify the comments metabox shows comments 21-12 (they are ssorted by recency).
6. Click on Show more comments.
7. Verify comments 11-2 are loaded (before this fix, comments 21-12 were loaded again).
8. Press Show more comments again and verify comment 1 is loaded (before this fix, the button would not be visible).

Steps to reproduce (WooCommerce)

1. Create a product.
2. In the frontend, add 21 reviews.
3. In the product editor, verify the reviews metabox shows reviews 21-12.
4. Click on Show more comments.
5. Verify reviews 11-2 are loaded.
6. Press Show more comments again and verify review 1 is loaded.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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[Aljullu]

There are some PHPUnit Tests automations failing, but they don't seem related.

[github-actions]

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Props aljullu.

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