Global Styles: Add pseudo-selector support for text inputs

[onemaggie]

Global Styles supports styling text inputs through theme.json, but it does not currently support the pseudo-classes and pseudo-element needed for common form states.

The Gutenberg implementation adds support for:

• :required
• :valid
• :invalid
• :focus
• :focus-visible
• ::placeholder

Pseudo-elements such as ::placeholder are output outside the :where() wrapper because pseudo-elements are not valid inside it.

Gutenberg PR: ​https://github.com/WordPress/gutenberg/pull/71634

This ticket tracks the WordPress Core backport for 7.2.

Backports the WP_Theme_JSON changes from ​Gutenberg PR #71634.

This adds :required, :valid, :invalid, :focus, :focus-visible, and ::placeholder support for the textInput element. Pseudo-elements are output without the :where() wrapper because pseudo-elements are not valid inside it.

## Testing

• php -l src/wp-includes/class-wp-theme-json.php
• vendor/bin/phpcs src/wp-includes/class-wp-theme-json.php

## Use of AI Tools

AI assistance: Yes
Tool(s): OpenAI Codex
Model(s): GPT-5
Used for: Translating the Gutenberg change to its WordPress Core counterpart, running validation, and drafting the pull request.