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

[rprichard]

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

[rprichard]

Previously mypy was complaining on this file:

$ mypy code-format-helper.py 
code-format-helper.py:29: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:29: note: Did you mean "List[...]"?
code-format-helper.py:36: error: "FormatHelper" has no attribute "friendly_name"  [attr-defined]
code-format-helper.py:44: error: "FormatHelper" has no attribute "instructions"  [attr-defined]
code-format-helper.py:90: error: "FormatHelper" has no attribute "friendly_name"  [attr-defined]
code-format-helper.py:94: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:94: note: Did you mean "List[...]"?
code-format-helper.py:123: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:123: note: Did you mean "List[...]"?
code-format-helper.py:132: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:132: note: Did you mean "List[...]"?
code-format-helper.py:135: error: Return value expected  [return-value]
code-format-helper.py:162: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:162: note: Did you mean "List[...]"?
code-format-helper.py:171: error: Bracketed expression "[...]" is not valid as a type  [valid-type]
code-format-helper.py:171: note: Did you mean "List[...]"?
code-format-helper.py:174: error: Return value expected  [return-value]
Found 11 errors in 1 file (checked 1 source file)

[tru]

LGTM

[rprichard]

I noticed that I forgot an annotation on DarkerFormatHelper.instructions. I added another commit to fix that. This file now passes mypy --strict code-format-helper.py, which does complain about functions that lack a return type annotation.