Missing type hints in divide_and_conquer/inversions.py

### What would you like to share?

The functions in `divide_and_conquer/inversions.py` are currently missing type hints.

Functions needing updates:
- `count_inversions_bf`
- `count_inversions_recursive`
- `_count_cross_inversions`

I would like to add these to improve readability and MyPy compliance.

### Additional information

_No response_