Normalize rich text block attribute schemas for REST validation

[chubes4]

Summary

• map block-only rich-text attributes to the REST-compatible string type at validation boundaries
• use the normalized schema for renderer-route validation, sanitization, and block render preparation
• add a renderer regression test that preserves rich-text markup

Testing

• php -l src/wp-includes/class-wp-block-type.php
• php -l src/wp-includes/rest-api/endpoints/class-wp-rest-block-renderer-controller.php
• php -l tests/phpunit/tests/rest-api/rest-block-renderer-controller.php
• PHPUnit dependencies are not installed in this worktree.

[github-actions]

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[github-actions]

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[chubes4]

Closing pending a deterministic, reduced-bootstrap reproduction and ownership evidence. No merge or deployment occurred.