fix: use stub instead of mock for unused handler

PHPUnit 13 warns about mock objects with no expectations. createStub() avoids the notice since the handler is never called in the expired-session test.

backend/tests/Unit/Middleware/AuthMiddlewareTest.php

@@ -125,7 +125,7 @@ class AuthMiddlewareTest extends TestCase
         );
 
         $request = $this->createRequestWithCookie('valid-token');
-        $handler = $this->createMock(RequestHandlerInterface::class);
+        $handler = $this->createStub(RequestHandlerInterface::class);
 
         $this->middleware->process($request, $handler);