test: add unit tests for argument_parser

[Jeong-Ryeol]

Addresses #158

Added unit tests for the argument_parser module.

Noticed there weren't any tests yet, so I started with argument_parser since it has clean, testable functions.

What's included:

• Basic test setup (conftest.py with fixtures)
• Tests for bool/list/dict conversion functions
• Tests for type annotation handling (Optional, Union types)
• Tests for the main convert_arguments function

Let me know if you'd like any changes!