REST API: Use wp_prepare_json_value_for_client() for view configs and entity fields

[jorgefilipecosta]

Follow up to https://core.trac.wordpress.org/changeset/64503 and https://core.trac.wordpress.org/changeset/64592.

This PR removes the private _rest_cast_empty_objects_from_schema() helper and uses the public wp_prepare_json_value_for_client() instead, so there is a single function that prepares empty objects according to a schema:

• The view config and entity fields endpoints call wp_prepare_json_value_for_client().
• WP_REST_View_Config_Controller::cast_empty_objects() (since 7.1) is kept and delegates to it.
• The private helper was added in this release cycle, so it is removed without deprecation.

wp_prepare_json_value_for_client() also handles cases the private helper missed: it recurses into the matching oneOf/anyOf branch (e.g. a form field children: [ [] ] is now [ {} ]), patternProperties, stdClass and JsonSerializable values, and keeps [] when the schema also allows an array. No view config or entity fields schema allows both an object and an array, so the existing responses are unchanged.

The change 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 -- --group entity-fields,view-config,json-schema

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[jorgefilipecosta]

Committed at 3673e3a.