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)
- Add folder_include parameter support in backend API handlers
- Add folder_include to FilterCriteria and implement multi-folder filtering logic
- Update frontend to send all include folders instead of only the first
- Add tests for single/multiple include folders, include with exclude, and non-recursive filtering
- Add blank line after module docstring for better PEP 8 compliance
- Reformat long lines to adhere to 88-character limit using Black-style formatting
- Improve string consistency by using double quotes consistently
- Enhance readability of complex list comprehensions and method calls
- Maintain all existing functionality while improving code structure
