Block hooks are not applied to plugin-registered templates using `register_block_template()`

[iamadisingh]

Block hooks are not being applied to templates registered by plugins using the Template Registration API (register_block_template()). This creates inconsistent behavior where block hooks work for theme templates and user-created templates, but not for plugin-registered templates.

Originally issue reported in GB: ​https://github.com/WordPress/gutenberg/issues/71139

Step-by-step reproduction instructions

1. Create a plugin.
2. Add this snippet to register a template and hook a block:

<?php
register_block_template(
    'my-plugin//plugin-template',
    array(
        'title'       => 'Test Template',
        'description' => 'This is a test template',
        'content'     => '<!-- wp:separator {"className":"is-style-dots"} --><hr class="wp-block-separator has-alpha-channel-opacity is-style-dots"/><!-- /wp:separator -->',
    )
);

add_action(
    'single_template_hierarchy',
    function () {
        return array( 'plugin-template' );
    }
);

add_filter(
    'hooked_block_types',
    function ( $hooked_block_types, $relative_position, $anchor_block_type, $context ) {
        if ( 'after' === $relative_position && 'core/separator' === $anchor_block_type ) {
            $hooked_block_types[] = 'core/post-title';
        }
        return $hooked_block_types;
    },
    10,
    4
);

3. Create a new page and add a Separator block
4. Visit a page (using theme template) and a post (using plugin template)
5. Notice the page correctly applies the block hook, while the post doesn't

Expected Result: Block hooks should be applied consistently to all template sources (theme, user-created, and plugin-registered).

Actual Result: Plugin-registered templates do not have block hooks applied, while theme templates and user-created templates do.

This PR fixes an inconsistency where block hooks were not being applied to templates registered by plugins using register_block_template().

Trac ticket: https://core.trac.wordpress.org/ticket/63808

Thanks for working on this, @Adi-ty! I tested with the testing steps from ​WordPress/gutenberg#71139 and I can confirm that:

1. In the frontend, the template from the plugin correctly displays the hooks:

Before After

2. When making a request to /wp-json/wp/v2/templates/twentytwentyfive//plugin-template, the template in the response now has the hooks applied to the content.

Before After

3. That is not a problem when accessing the template in the site editor (ie: /wp-admin/site-editor.php?canvas=edit&p=%2Fwp_template%2Ftwentytwentyfive%2F%2Fplugin-template). Hooks are still rendered only once.

I'm approving from my side, but I don't have permisions in WP core. @ockham wondering if you (or somebody you know) can help us move this forward?

Thanks for testing and confirming the changes. Hopefully we can get this moved forward soon.

Thank you both! I'll go ahead and commit this 👍

[Bernhard Reiter]

In 60688:

Block Hooks: Apply block hooks to plugin-registered templates.

As of WordPress 6.7.0, it is possible -- e.g. for plugins -- to register block templates via a new API, register_block_template(). Unlike block templates loaded from theme files or from the database, however, those block templates didn't have Block Hooks applied. This changeset rectifies this inconsistency.

Props iamadisingh, aljullu, bernhard-reiter.
Fixes #63808.