fix: resolve typing_audit violations in cli_test_helper

- Inherit _StdinWithBuffer and _StdoutCapture from io.StringIO so they are proper TextIO types — eliminates type: ignore[assignment] and type: ignore[arg-type] on redirect_stdout / sys.stdin assignment - Type _cli parameter as None (all tests pass None post-migration) - Remove **_kwargs catch-all; drop stale obj= kwarg from one test call - Drop `from typing import Any` — file is now Any-free

typing_audit: 0 violations mypy: 0 errors pytest: 3202 passed