fix: add missing return type to getAssociatedModel

src/ItemCollection.php

@@ -56,7 +56,7 @@ public function __get($name)
      *
      * @return mixed
      */
-    protected function getAssociatedModel()
+    protected function getAssociatedModel(): mixed
     {
         if (! $this->has('associatedModel')) {
             return null;