ruff

99d97bd7 - [ty] Tighten up a few edge cases in `Concatenate` type-expression parsing (#24172)

[ty] Tighten up a few edge cases in `Concatenate` type-expression parsing (#24172) ## Summary There were: - a couple of places where we emitted two diagnostics where one would do - a few places where we fell back to overly precise inferred types in error cases (inferring types like `(Unknown, /) -> None` where `(...) -> None` would arguably be better) - a few places where we correctly emitted a diagnostic, but didn't have an ideal error message This PR just makes some minor cleanups in that area ## Test Plan new mdtests