Expand rules_python's type checking in CI to apply to remaining code

...where type errors are still prone to getting introduced.

Quoting @aignas in https://github.com/bazel-contrib/rules_python/pull/3240#issuecomment-3384929943
> I personally am not against doing type checking against all of the python code within the repo, but that is a fair amount of work that would definitely benefit from community also pitching in. I'll create a place-holder ticket that is reasonably wide (and we already have a CI job that is doing type checking on the runfiles library), so initially we could use something similar in our initial approach.

Start by reviewing the existing CI job that type-checks the runfiles library and identify the remaining Python code in the repository that is not covered. The work is done when CI applies type checking to that remaining code without introducing unresolved type errors.

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