[Workflow] make code-format-helper.py mypy-safe (NFC) (#69691)

Fix type errors that mypy reports with code-format-helper.py.

Add a few return type annotations and change `param: [str]` to
`param: list[str]`.

Leave a few required FormatHelper members missing instead of defining a
placeholder:
 - FormatHelper.name
 - FormatHelper.friendly_name
 - FormatHelper.format_run: NotImplementedError() instead of `pass`