Add comprehensive tests for User Preferences Service

- Test all IUserPreferencesService methods (get, set, setMany, delete, getAll, has, clear, listEntries)
- Test UserFavoritesService (add, remove, has, toggle, list)
- Test prefix filtering and multi-user isolation
- Test different value types (scalar, object, array, null)
- Use in-memory IDataEngine stub for fast unit tests

Agent-Logs-Url: https://github.com/objectstack-ai/framework/sessions/6c82c1b8-239c-4a91-8794-7e22d1fb5fdc

Co-authored-by: hotlong <50353452+hotlong@users.noreply.github.com>

Author: Claude