Write Unit Tests and Ingtegration Tests

### Feature Description

Write unit tests and integration tests using `pytest`.

### Reasons

Tests help ensure code correctness and compatibility as the application grows.

### Checklist

- [x] Tests for `corpus`
- [x] Tests for `lexicon`
- [x] Tests for `utils`
- [ ] Overall Integration Tests
    - [ ] Tests for `cli`
    - [ ] Tests for `shell`
