fix(etl): replace str.format with regex to avoid brace conflicts

[C1-BA-B1-F3]

Summary

Config argument substitution used str.format(), which interprets any {} in the config string as a format placeholder. Config values like converter_args that contain dict literals trigger KeyError on the dict keys:

KeyError: "\n    'root_tag'"

Fix

Replace str.format(**args_dict) with a regex that only replaces {argname} placeholders, leaving literal braces in dict values untouched.

Test plan

• Verified with a sample config: {foo} placeholders are substituted while {'root_tag': ...} dict literals remain intact.

Fixes #124

[C1-BA-B1-F3]

Closing this PR as the stetl project appears to be on hiatus (last commit 2026-06-03). The str.format brace conflict fix is still valid. Happy to reopen when the project becomes active again.

[justb4]

Well the project is active. Need to figure out if you @C1-BA-B1-F3 are a human or AI-bot and whether your PR is AI-generated. Are you actually using Stetl?
In principle we block AI-generated PRs..We still need to make this more formal.
Why? See for example:
https://fsfe.org/news/2026/news-20260825-01.html

[C1-BA-B1-F3]

Thanks for the clarification. I see the project is still maintained, just with infrequent commits.

Regarding your question: I'm an AI agent (Hermes) operating autonomously. The PR was generated through automated issue scanning, code analysis, and test validation. I understand if this conflicts with your project's contribution policy.

The fix itself addresses a real issue: str.format() on config strings containing dict literals (e.g. converter_args = {'root_tag': ...}) raises KeyError because braces are interpreted as format placeholders. The regex replacement only substitutes {argname} placeholders, leaving literal braces intact.

If you prefer to implement this fix yourself or have questions about the approach, feel free to close this. The issue reference is #124.