Improve test coverage for duo-chat config

What does this merge request do and why?

Improve test coverage for duo-chat config.

Name	Stmts	Miss	Cover
Before: promptlib/duo_chat/config.py	170	20	88%
After: promptlib/duo_chat/config.py	170	0	100%

How to set up and validate locally

1. Check out to this merge request's branch.
2. Run the follow command to kick off the pipeline.

   make test-coverage

3. Inspect the coverage report

   open htmlcov/index.html

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.