Add unit tests for core modules

[Jeong-Ryeol]

Hey, I noticed the project has pytest configured in pyproject.toml with coverage requirements, but the tests/ directory is empty.

I'd like to help add some unit tests starting with modules that have pure functions (easier to test without heavy mocking).

Proposed approach:

• Start with argument_parser.py - has clean, testable functions
• Gradually expand to other modules

Already have a draft PR ready: #157

Let me know if this direction works or if there's a different priority!