fix: ignore type errors in copied files without mutants

[skyswordw]

Summary

Fixes a type_check_command crash when a type checker reports an error in a copied file that contains no generated mutmut mutant functions.

The type-check filtering step still raises for unowned errors in files that do contain generated mutants, because those remain ambiguous and may indicate a mutation affected code outside the mutant function body.

Closes #522.

Testing

• uv run pytest tests/test_type_checking.py
• uv run pytest tests/test_type_checking.py tests/e2e/test_e2e_type_checking.py
• uv run ruff check .
• uv run ruff format --check .
• uv run pytest -k 'not test_setproctitle_crashes_after_fork_with_corefoundation_loaded' -> 299 passed, 2 skipped, 1 deselected

Notes:

• uv run pytest reported 299 passed, 2 skipped, 1 failed; the failure was the existing macOS sentinel tests/utils/test_safe_setproctitle.py::test_setproctitle_crashes_after_fork_with_corefoundation_loaded, unrelated to type-check filtering.
• uv run pre-commit run --all-files was attempted but hung during hook environment bootstrap in git submodule update --init --recursive --depth=1, before running project checks.

[nicklafleur]

@skyswordw thanks for contributing a detailed issue and opening this draft. I've enabled test runs for you to help, feel free to request a review from me when the branch is ready.

[skyswordw]

Thanks for enabling the test runs. The PR is ready now, and the Python 3.10-3.14 jobs are all green.

[nicklafleur]

LGTM 🚀

[skyswordw]

Thanks for reviewing and merging it.