Closes #75670 (Still WIP exploration)

What?

Closes
Adds a Post Types column to the Templates DataViews list, showing which post types each template is associated with (as defined via postTypes in theme.json custom templates).

Why?

Template postTypes data is already defined in theme.json and used internally for filtering, but it was never exposed in the REST API or surfaced in the Site Editor's template list UI. This makes it difficult for users to see at a glance which templates are available for which post types.

How?

WIP - Needs discussion on proper way we could get this backported.

• Gutenberg_REST_Templates_Controller_7_0: Added post_types to prepare_item_for_response() — returns the template's post_types array (or an empty array if not set). Added get_item_schema() to register post_types as a readonly array-of-strings field.
• Gutenberg_REST_Static_Templates_Controller: Extended get_item_schema() to include the same post_types property for the registered-templates endpoint.
• template-activate.php: After building a template from file via _build_block_template_result_from_file(), the postTypes data from the theme file is now propagated onto the template object so it's available for serialization.

Testing Instructions

1. Activate a theme that defines custom templates with postTypes in its theme.json (e.g., a theme with entries like "postTypes": ["post"] or "postTypes": ["post", "page"] under customTemplates).
2. Navigate to Appearance → Editor → Templates.
3. Switch to Table layout.
4. Verify a "Post Types" column appears showing the associated post types for each template.
5. Templates with postTypes defined should show the types as a comma-separated list (e.g., post or post, page).
6. Templates without postTypes defined should show an em dash (—).
7. Switch to a theme that does not define any postTypes on its custom templates.
8. Verify the "Post Types" column does not appear at all.

Testing via REST API

1. Make a GET request to /wp/v2/templates (or /wp/v2/registered-templates).
2. Verify each template object includes a post_types field.
3. Custom templates with postTypes in theme.json should return the array (e.g., ["post"]).
4. Templates without postTypes should return an empty array ([]).

Testing Instructions for Keyboard

1. Navigate to the Templates list using keyboard (Tab / arrow keys).
2. Switch to Table layout via the Layout button.
3. Verify the "Post Types" column is reachable and readable via screen reader / keyboard navigation.
4. Column values should be announced correctly (comma-separated post type names, or "—" for empty).

Screenshots

Table View

Grid View

List View