Abilities API: Add coverage for top-level required in input validation

[gziolo]

Summary

Adds focused test coverage documenting how a top-level required keyword is treated during WP_Ability::validate_input(), and removes inert top-level required flags from existing test schemas.

Background

WordPress validates object values against either a draft-04 required array of property names, or a draft-03 per-property required boolean (see rest_validate_value_from_schema()). A required flag placed at the root of a schema is never consulted by the validator, so it has no effect on validation. A number of test schemas in this file set required => true at the top level of simple-type schemas, which is inert and misleading.

Changes

• New data-provider-driven test_validate_input_top_level_required, covering both valid and invalid input for each variant:
  • top-level required: true — inert (only type is enforced)
  • top-level required: false — inert; does not permit null
  • required omitted — baseline; identical behavior
  • draft-04 required array on an object type — honored; enforces property presence
• Removes the inert top-level required => true flags from simple-type schemas. The meaningful per-property required flags in the object data set are preserved.

This is a test-only change: it validates existing WP_Ability behavior and adds no production code.

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

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