REST API: add WP_REST_Posts_Controller route to fetch page counts

[ramonjd]

What?

Alternative to #66294

Extends WP_REST_Posts_Controller with a new route to fetch page counts

This PR only affects the page post type.

In the Core sync, all post types that inherit from WP_REST_Posts_Controller will get the /count route.

Why?

The need for a performant post counts check for the block editor. See: #65223 (comment)

Contrast this with $query->found_posts which returns the total count for a specific query only and would degrade performance given n posts.

How?

Via wp/v2/pages/count, the editor can fetch the total number of post counts.

This is done using wp_count_posts.

Testing Instructions

Run tests:

npm run test:unit:php:base -- --filter Gutenberg_Test_REST_Posts_Controller

Examples to run in the browser console (in the editor):

await wp.apiFetch( { path: '/wp/v2/pages/count' } )

/*
{
    "publish": 1,
    "future": 0,
    "draft": 2,
    "pending": 0,
    "private": 0,
    "trash": 0,
    "auto-draft": 0
}
*/

Also add a custom post status and ensure it appears in the response:

/**
 * Create initial post types.
 */
function create_initial_post_status_test() {
	register_post_status( 'ridgy_didge', array( 'public' => true ) );
}

add_action( 'init', 'create_initial_post_status_test', 0 ); // Highest priority

await wp.apiFetch( { path: '/wp/v2/pages/count' } )

/*
{
    "publish": 1,
    "future": 0,
    "draft": 2,
    "pending": 0,
    "private": 0,
    "trash": 0,
    "ridgy_didge": 0,
    "auto-draft": 0
}
*/

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Co-authored-by: ramonjd <[email protected]>

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[ramonjd]

If we ever come back to this, I believe #66294, the original implementation, is better.

See: #66294 (comment)