- Added `common.py` to handle shared validation logic for line items, including checks for invoice existence, class validity, and customs data.
- Introduced `create.py` for validating new line items, ensuring required fields are present and calculating costs based on currency type.
- Created `update.py` to manage partial updates of line items, maintaining existing values when new data is not provided.
- Enhanced error handling with specific messages and solutions for various validation failures.