Refactor: Change to dataclass, Move files, Refactor test codes and Fix mypy

[VictorTan93]

📝 Description

Summary

This refactor-only PR improves code organization, type safety, and maintainability. The changes are entirely internal and maintain backward compatibility.

1. Converted Money and Currency classes to use Python's @dataclass decorator
2. Fix some mypy
3. Move Money and Currency into its own file in dataclasses folder
4. Move CURRENCY into its own file constants.py
5. Move all custom exceptions into its own file exceptions.py
6. Split monolithic test files into focused modules (e.g., test_money_operations.py, test_django_lookups.py)
7. Change to function-based test codes

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✅ Pre-review checklist for the PR author

• I have tested my code locally on my machine
• I have written tests (if appropriate)
• I have added appropriate label to PR

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