Fix cursor positioning in collaborative cell writing

- Fixed cursor lagging one index behind by using current position instead of original diff indices
- Added final cursor position update to ensure cursor ends after last character
- Improved function signatures to use delete_length instead of old_start/old_end
- Enhanced docstrings and code documentation for better maintainability
- Added comprehensive unit tests for collaborative writing functionality
- All tests pass, ensuring robust cursor positioning across various scenarios

Author: Zsailer