Summary
Move type-only imports in optuna/visualization/_param_importances.py into a TYPE_CHECKING block.

Motivation

• Limits runtime imports by isolating type-only dependencies under TYPE_CHECKING.
• Clarifies the boundary between imports required at runtime and those used solely for type checking.
• The implementation follows the guideline proposed in issue Use TYPE_CHECKING if necessary #6029.

Changes

• Moved type-only imports (Callable, BaseDistribution, BaseImportanceEvaluator, Study, FrozenTrial) into if TYPE_CHECKING:.

Verification

• Existing visualization tests passed.
• Pre-commit checks (ruff, formatting, mypy) passed.