Posts, Post Types: Ensure all entries in the list returned by wp_parse_list() are scalar.

This changeset fixes a warning where `strip_tags()` expected its first parameter to be a string rather than an array. It contains the following changes:

- Removal of sanitize_callback to allow the REST API to handle the validation natively, this also causes the proper error to be output for `?slug[0][1]=2` that it's an invalid value.
- Ensure that `wp_parse_list()` only returns a single-dimensioned array, even if passed a multi-dimension array, which fits the functions expected use case and resolves warnings in code that expects the function to return a single-dimensioned array.

Props dd32, TimothyBlynJacobs.
Fixes #55838.


git-svn-id: https://develop.svn.wordpress.org/trunk@54476 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -4821,6 +4821,9 @@ function wp_parse_list( $list ) {
 		return preg_split( '/[\s,]+/', $list, -1, PREG_SPLIT_NO_EMPTY );
 	}
 
+	// Validate all entries of the list are scalar.
+	$list = array_filter( $list, 'is_scalar' );
+
 	return $list;
 }