Abilities API: Normalize required schema shape for REST responses

[gziolo]

Summary

Normalizes the required keyword in an ability's input and output schemas to its JSON Schema draft-04 form when they are exposed through the REST API (wp-abilities/v1).

Background

WordPress validates objects against either a draft-04 required array of property names on the object schema, or a draft-03 per-property required boolean (see rest_validate_value_from_schema()). Both work internally, but only the array form is valid JSON Schema draft-04. Ability schemas registered with the per-property boolean form (as shown in the wp_register_ability() examples) were therefore exposed to clients in a shape that does not conform to the draft-04 contract the endpoint advertises via $schema.

Changes

In WP_REST_Abilities_V1_List_Controller::prepare_schema_for_response():

• Collect per-property required: true booleans and emit them as a required: [ ... ] array of property names on the parent object schema — applied recursively, so nested objects each get their own array.
• Strip required: false and any boolean required that has no draft-04 equivalent (for example on a scalar root schema).
• Preserve and merge with an existing draft-04 required array, de-duplicating names.

The transformation only rewrites the REST response copy; server-side validation continues to use the stored schema via validate_input(), so behavior is unchanged. Adds test coverage for the conversion: top-level, nested objects, objects nested inside array items, required: false, mixed draft-04 array + draft-03 booleans, and the scalar-root case.

Trac ticket: https://core.trac.wordpress.org/ticket/64955

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