REST API: Add source parameter for types endpoint

[priethor]

What

• Added a new source parameter to the /wp/v2/types REST API endpoint that allows filtering post types by their origin: core (WordPress built-in), scf (for SCF managed types), or other for the rest of CPTs.
• Works with both collection (/wp/v2/types) and single item (/wp/v2/types/{type}) endpoints
• Properly documented in the OpenAPI schema

Why

When working on the Command Palette Integration and considering using the REST API to define CPT-related commands, requesting specifically for the CPTs managed by SCF became a need, and it will be needed, too, when implementing DataViews.

In fact, this branch is directly based on #124 and started in parallel, as we both realized we needed to tweak the Types endpoint if we wanted to leverage the API.

How

• If the source parameter is not defined, nothing changes.
• For specific, single-type requests:
  • Hooks to rest_request_before_callbacks for the single request, returning 404 if the requested type doesn't meet the criteria.
• For collection requests:
  • Hooks to rest_prepare_post_type to filter each post type by source before WordPress prepares it.
  • Hooks to rest_pre_echo_response to remove null values returned by rest_prepare_post_type in the response array.
  • Since the filter rest_prepare_post_type fires once per post type, the Post Types belonging to the requested source are calculated in advance in rest_request_before_callbacks and reused throughout the whole request.

Testing Instructions

1. API Endpoint Testing:
   - Make a GET request to /wp/v2/types to see all post types
   - Add ?source=core to filter for WordPress built-in post types only
   - Add ?source=scf to see only post types managed by SCF
   - Try ?source=other to see post types registered from other sources
   - Test single post type endpoints like/wp/v2/types/post?source=core
2. Validation Testing:
   - Try an invalid source value (e.g., ?source=invalid) to verify validation works
   - Confirm a 404 error when requesting a post type with the wrong source (e.g., /wp/v2/types/post?source=scf`
3. Unit and E2E Tests (heavily assisted by AI):
   - Run PHPUnit tests: composer test:php -- --filter=Test_REST_Types_Endpoint
   - Run E2E tests: npm run test:e2e tests/e2e/rest-api-types-endpoint.spec.ts

Potential follow-ups

• Add a transient cache for the post type source calculation.
• With the transient cache in place, return the post type source as a field in the response, as there would be no performance loss.
• Add a hook so that other plugins can register their own sources.

[cbravobernal]

In my opinion, most of console logs are not necessary. Let's avoid to fill the logs of CI with them. The rest of the PR is working as expected, nice job!! 🚀