fix(types): resolve pre-existing basedpyright errors in tests

Fix ~790 basedpyright errors across the test suite:
- Type stub subclasses of real production classes with super().__init__()
- Add missing generic type arguments and Dict[str, Any] annotations
- Add None guards before subscript/member access
- Adapt tests to production API changes (removed dead handlers,
  PersistentModelCache.get_default, _i18n_filter_added location)

tests/integration/test_download_flow.py

@@ -193,7 +193,7 @@ class TestDownloadRouteIntegration:
 
         assert response.status == 400
         # Response might be JSON or text, check both
-        if hasattr(response, 'text'):
+        if hasattr(response, 'text') and response.text is not None:
             error_text = response.text.lower()
         else:
             body = response.body