Incompatible types when running mypy hook

### Motivation

mypy pre-commit hook fails due to int type errors.
 
- `optuna/study/_multi_objective.py`
- `optuna/_gp/optim_mixed.py`

![Image](https://github.com/user-attachments/assets/a4964edf-c1d2-4a11-9b96-8cba424f1ecd)

### Suggestion

Fix these by adding explicit types

### Additional context (optional)

_No response_