Add runtime type checking (e.g. typeguard) during test runs

[toumorokoshi]

There are currently some type mismatches that are not caught by mypy and static typing.

There was a previously pull request #316 that attempted to address this by adding typeguard. However, adding such a tool comes with significant work on massaging the types to be correct, and so had to be abandoned due to other priorities.

It would be fantastic work to re-integrate this type checking into the repo.

[github-actions]

This issue was marked stale due to lack of activity. It will be closed in 30 days.