fix: TypeError when there is numeric field name

TypeError: str_contains(): Argument #1 ($haystack) must be of type string, int given

system/Validation/Validation.php

@@ -163,6 +163,9 @@ public function run(?array $data = null, ?string $group = null, $dbGroup = null)
         // Run through each rule. If we have any field set for
         // this rule, then we need to run them through!
         foreach ($this->rules as $field => $setup) {
+            //  An array key might be int.
+            $field = (string) $field;
+
             $rules = $setup['rules'];
 
             if (is_string($rules)) {