[Task] Refactor code to fix existing type checker errors

[Apricot-S]

Summary

Several parts of the codebase currently produce type errors.
This task focuses on refactoring or adjusting type annotations so that the project type‑checks cleanly.

Scope

• Decide whether to use mypy or ty
• Identify existing type errors
• Refactor code or update annotations to resolve them
  • Avoid behavioral changes unless required to fix typing issues
• Add type check in CI

Notes

This issue is intentionally broad and does not require detailed enumeration of each error.
Individual fixes can be handled in separate PRs if needed.

[Nihisil]

@Apricot-S what do you think about using ty instead of mypy?