Fix rich-text attribute type normalization in WP_Block_Type

[USERSATOSHI]

Trac ticket: https://core.trac.wordpress.org/ticket/61406

Normalize rich-text block attribute type to string in WP_Block_Type::set_props()

Block attributes with type: "rich-text" (used in block.json for the editor) are not valid JSON Schema types. When WP_Block_Type::prepare_attributes_for_render() calls rest_validate_value_from_schema(), the unrecognised rich-text type triggers a _doing_it_wrong() notice:

The "type" schema keyword for content can only be one of the built-in types: array, object, string, number, integer, boolean, and null.
This affects all core blocks that define attributes as rich-text: paragraph, heading, list-item, image (caption), embed (caption), quote, pullquote, verse, code, preformatted, table, gallery, button, file, video, audio, details, and accordion-heading.

Changes in this PR:

1. src/wp-includes/class-wp-block-type.php — In set_props(), normalise rich-text to string at block registration time. This runs once (not per-render) and occurs before the register_block_type_args filter, so plugins can still override if needed.
2. tests/phpunit/tests/blocks/wpBlockType.php — Added a richtext attribute case to test_prepare_attributes() to verify that rich-text typed attributes pass validation without triggering _doing_it_wrong.

Related: Gutenberg issue WordPress/gutenberg#72180

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