[MAINTENANCE] Currency Script | Expand Test Coverage, Add CLI (main.py), and Improve Docs

[sue-lim]

This is an update to the previous [FEATURE] Currency Script | Add unit tests for core modules issue. Expanding the scope to not only add unit tests, but also:

• Implement a simple CLI via main.py
• Add/ensure minimal docstrings across modules
• Update README.md and requirements.txt

Details

Ensure unittest coverage for:

• api_handler.py
• converter.py
• currencies.py

Create a simple CLI script in main.py:

• Prompt user for currency input and amount

• Show converted amount

• Option to perform another conversion or exit

• Ensure all modules (api_handler, currencies, converter) have simple, readable docstrings

• Update README.md with usage, setup, and test instructions

• Update requirements.txt with dependencies (requests, etc.)

Rationale
There are currently no automated tests. This task improves long-term maintainability, reduces risk of regressions, and introduces a user-friendly way to run the tool.

[sue-lim]

@DhanushNehru - please kindly assign this issue to me.
Many thanks