chore: fix mypy warnings ai_gateway root folder

What does this merge request do and why?

This fixes mypy warnings in ai_gateway and tests root folder. This also adds an exclude list to allow fixing these errors incrementally.

Contributes to #436

How to set up and validate locally

1. Check out to this merge request's branch.
2. Check that there is no mypy errors when running the standard linting job make check-mypy.
3. Check that you can see outstanding errors when running make check-mypy TODO=true.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.