[Py Reformat] Exclude third-party in code-format-helper.py (#83872)

Follow-up from PR #83491. `Darker`'s configuration is ignored because of the way we invoke it - with an explicit list of files. We need to filter it in `code-format-helper.py`.

Author: mtrofin

llvm/utils/git/code-format-helper.py

@@ -125,6 +125,7 @@ def update_pr(self, comment_text: str, args: FormatArgs, create_new: bool) -> No
             pr.as_issue().create_comment(comment_text)
 
     def run(self, changed_files: List[str], args: FormatArgs) -> bool:
+        changed_files = [arg for arg in changed_files if "third-party" not in arg]
         diff = self.format_run(changed_files, args)
         should_update_gh = args.token is not None and args.repo is not None