Block Hooks API: Move hooked blocks injection logic

[ockham]

In a similar vein as @tjcafferkey's WordPress#6604, this introduces a new insert_hooked_blocks_into_rest_response filter and adds it to the rest_prepare_wp_navigation hook. (Note that this branch is actually based on Tom's; the reason is that it uses at least one function that's introduced there.)

This is part of an effort to move code from the Navigation block into Core. Specifically, insert_hooked_blocks_into_rest_response is based on block_core_navigation_insert_hooked_blocks_into_rest_response.

Testing instructions:

Preparation: Make sure you're using a Block Theme to test, ideally TT4. Add the following code to your site (e.g. to your theme's functions.php).

function register_logout_block_as_navigation_last_child( $hooked_blocks, $position, $anchor_block, $context ) {
	if ( $anchor_block === 'core/navigation' && $position === 'last_child' ) {
		$hooked_blocks[] = 'core/loginout';
	}

	return $hooked_blocks;
}

add_filter( 'hooked_block_types', 'register_logout_block_as_navigation_last_child', 10, 4 );

1. If there's an existing wp_navigation post object for you Navigation menu in your database, make sure that it doesn't have any _wp_ignored_hooked_blocks post meta set. (You can check the latter via npm run wp-env run cli wp post meta list <postId>.) Otherwise, you'll need to delete said post meta.
2. Go to the site editor and verify that the Login/out button block is added to the Navigation menu. You will probably see two copies. The reason is that the Navigation block code still keeps inserting hooked blocks on its own.
3. To avoid the latter, apply the following patch, and reload the editor. You should now see only one copy of the Login/out block added.

diff --git a/src/wp-includes/blocks/navigation.php b/src/wp-includes/blocks/navigation.php
index ba1644cf60..4a62295c09 100644
--- a/src/wp-includes/blocks/navigation.php
+++ b/src/wp-includes/blocks/navigation.php
@@ -1566,10 +1566,3 @@ function block_core_navigation_insert_hooked_blocks_into_rest_response( $respons
  */
 $rest_prepare_wp_navigation_core_callback = 'block_core_navigation_' . 'insert_hooked_blocks_into_rest_response';
 
-/*
- * Injection of hooked blocks into the Navigation block relies on some functions present in WP >= 6.5
- * that are not present in Gutenberg's WP 6.5 compatibility layer.
- */
-if ( function_exists( 'set_ignored_hooked_blocks_metadata' ) && ! has_filter( 'rest_prepare_wp_navigation', $rest_prepare_wp_navigation_core_callback ) ) {
-       add_filter( 'rest_prepare_wp_navigation', 'block_core_navigation_insert_hooked_blocks_into_rest_response', 10, 3 );
-}

I will follow up with a Gutenberg PR to make the Navigation block's insertion of hooked blocks conditional. That other PR will have to get merged and sync'd to Core before we can land this change.

Trac ticket: https://core.trac.wordpress.org/ticket/60759

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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.

[ockham]

Moved to WordPress#6677.