Abilities API: Encode empty object results as JSON objects

[jorgefilipecosta]

Follow up to https://core.trac.wordpress.org/changeset/64592 and #14139.

What

• The run endpoint prepares every result for JSON clients with wp_prepare_json_value_for_client(), as the AI client already does, not only associative arrays. An empty array returned for an object output schema is now encoded as {} at the root, in lists, and inside object or JsonSerializable results, instead of [], which the JS client (executeAbility()) rejected with ability_invalid_output. This fixes it for all abilities, e.g. core/settings-get when no setting matches.
• Arrays stay arrays: an empty list, a list of strings, or an empty array for a schema that also allows an array are still encoded as [].
• The global _fields and _embed parameters, and response links, no longer fail with a fatal error on object data:
  • _rest_array_intersect_key_recursive() (_fields) and WP_REST_Server::embed_links() (_embed) leave non-array data unchanged. An empty object has no fields to filter and no links to embed.
  • WP_REST_Server::response_to_data() converts object data to an array before adding _links.
• This also fixes the fatal from https://core.trac.wordpress.org/ticket/66230, e.g. /wp/v2/taxonomies?type=wingding&_fields=name now returns {}.

await wp.apiFetch( { path: wp.url.addQueryArgs( '/wp-abilities/v1/abilities/core/settings-get/run', { input: { group: 'writing', fields: [ 'title' ] } } ) } );
// Before: []
// After:  {}

await wp.apiFetch( { path: wp.url.addQueryArgs( '/wp-abilities/v1/abilities/core/settings-get/run', { input: { group: 'writing', fields: [ 'title' ] }, _fields: 'title' } ) } );
// After:  {}

Issue found while testing the query abilities through the JS client; the fix was drafted with AI assistance and reviewed by me.

Trac ticket: https://core.trac.wordpress.org/ticket/66267

Testing

Verify unit tests are passing:

npm run test:php -- --filter 'test_output_is_prepared_for_json_clients|test_no_matching_setting_is_encoded_as_an_empty_object|test_rest_filter_response_fields_empty_objects|test_link_embedding_empty_objects'

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[gziolo]

I noted in #14139 that we should follow-up and figure out how to map [] to {} in a way that doesn't break REST API handler when global handlers like _fields or _links get passed. Casting to (object) in the ability doesn't resolve the problem.