REST API: Reject non-string custom CSS in the global styles controller

[ramonjd]

styles.css has no type constraint in the global styles request schema, so a REST consumer can PUT an array (or any non-string value). That value reaches strlen() in validate_custom_css() and throws an uncaught TypeError on PHP 8+.

Guard validate_custom_css() against non-string input, returning a rest_custom_css_invalid_type WP_Error with a 400 status instead of a fatal.

Adds a controller test that PUTs styles.css as an array and asserts the 400 response, plus unit tests covering array, integer, boolean, null, and object values.

Backports WordPress/gutenberg#80338

Trac ticket: https://core.trac.wordpress.org/ticket/65640

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[andrewserong]

This LGTM and matches the GB PR. As mentioned on the other PR, I think throwing a 400 in this case is the right behaviour as any consumer should be following the theme.json schema for this, where css is defined as a string. And throwing a 400 is safer than trying to infer a value from differently shaped data.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 62762
GitHub commit: 4c7af09

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.