Add exists method to WebdavControllerTest

Included a new method `exists` to handle existence checks for StoredObject or StoredObjectVersion within WebdavControllerTest. This method currently always returns true, ensuring initial compatibility and providing a foundation for future logic refinements.

src/Bundle/ChillDocStoreBundle/Tests/Controller/WebdavControllerTest.php

@@ -446,4 +446,9 @@ class MockedStoredObjectManager implements StoredObjectManagerInterface
     public function clearCache(): void {}
 
     public function delete(StoredObjectVersion $storedObjectVersion): void {}
+
+    public function exists(StoredObject|StoredObjectVersion $document): bool
+    {
+        return true;
+    }
 }