lint: fix type errors at variable assignments

We have this set in our pyproject.toml

[tool.mypy]
disable_error_code = [
    # https://github.com/python/mypy/issues/6232
    "assignment",
]

The linked issue https://github.com/python/mypy/issues/6232 has been closed on Mar 19, 2025 by https://github.com/python/mypy/pull/18727 . We can use that work at this point.

I tried running mypy with the mentioned flags on our code --allow-redefinition-new --local-partial-types, but it reduces the number of "assignment" errors from 149 to 122, making me think we have our own issues that are not related to the mypy check.