refactor: add return types to __toString()

See https://psalm.dev/docs/running_psalm/issues/MethodSignatureMustProvideReturnType/

Author: kenjis

system/Cookie/Cookie.php

@@ -633,7 +633,7 @@ public function toHeaderString(): string
     /**
      * {@inheritDoc}
      */
-    public function __toString()
+    public function __toString(): string
     {
         $cookieHeader = [];
 

system/View/Cells/Cell.php

@@ -118,7 +118,7 @@ final protected function view(?string $view, array $data = []): string
     /**
      * Provides capability to render on string casting.
      */
-    public function __toString()
+    public function __toString(): string
     {
         return $this->render();
     }