chore: Enable ty `unused-ignore-comment` rule (#762)

## Summary

Now that `ty` supports detecting unused suppression comments (upstream
issues [astral-sh/ty#278](https://github.com/astral-sh/ty/issues/278)
and [astral-sh/ty#1501](https://github.com/astral-sh/ty/issues/1501) are
closed), enable the `unused-ignore-comment` rule project-wide so
obsolete `# ty: ignore[...]` comments are caught automatically.

Verified that all current `# ty: ignore[...]` suppressions in the
codebase are still required (none had to be removed).