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/services/test_backup_service.py

@@ -209,8 +209,8 @@ async def test_model_update_service_migrates_legacy_snapshot_db(tmp_path, monkey
             return str(legacy_db)
 
     monkeypatch.setattr(
-        "py.services.persistent_model_cache.get_persistent_cache",
-        lambda *_args, **_kwargs: LegacyCache(),
+        "py.services.persistent_model_cache.PersistentModelCache.get_default",
+        lambda *args, **kwargs: LegacyCache(),
     )
 
     service = ModelUpdateService(settings_manager=DummySettingsManager())