REST API: Add uniqueItems as a permitted endpoint argument JSON Schema keyword.

Support for the `uniqueItems` keyword was added in [48357], but wasn't included in the list of permitted keywords for `WP_REST_Controller::get_endpoint_args_for_item_schema()`. This prevented the keyword from being automatically supported if specified in a controller's item schema.

Fixes #51021.


git-svn-id: https://develop.svn.wordpress.org/trunk@48796 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api/endpoints/class-wp-rest-controller.php

@@ -645,6 +645,7 @@ abstract class WP_REST_Controller {
 			'pattern',
 			'minItems',
 			'maxItems',
+			'uniqueItems',
 		);
 
 		foreach ( $schema_properties as $field_id => $params ) {