REST API: Ensure args is an array of arrays in register_rest_route().

When calling `register_rest_route()`, the `args` parameter for a route should be an array of arrays.  However, some plugins/themes have passed an array of strings or key-value pairs which produces a PHP warning when `array_intersect_key` is used to filter the array keys based on an allowed list of schema keywords.

This change adds a check of the `args` parameter to ensure it's an array of arrays, presenting a `_doing_it_wrong` if any element of `args` is not an array and restructuring to an array of arrays.  This change also adds a unit test for the incorrect usage described above, expecting that a `_doing_it_wrong` is produced.

Props slaFFik, desrosj, apermo, AndrewNZ, aristath, poena, dovyp, timothyblynjacobs, Hinjiriyo, johnmark8080, nateallen.
Fixes #51986.

git-svn-id: https://develop.svn.wordpress.org/trunk@54339 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -103,6 +103,18 @@ function register_rest_route( $namespace, $route, $args = array(), $override = f
 				'5.5.0'
 			);
 		}
+
+		if ( count( array_filter( $arg_group['args'], 'is_array' ) ) !== count( $arg_group['args'] ) ) {
+			_doing_it_wrong(
+				__FUNCTION__,
+				sprintf(
+				/* translators: %s: The REST API route being registered. */
+					__( 'REST API $args should be an array of arrays. Non-array value detected for %s.' ),
+					'<code>' . $clean_namespace . '/' . trim( $route, '/' ) . '</code>'
+				),
+				'6.1.0'
+			);
+		}
 	}
 
 	$full_route = '/' . $clean_namespace . '/' . trim( $route, '/' );