chore: fix mypy warnings in auth/

What does this merge request do and why?

This MR fixes all mypy warnings on ai_gateway/auth folder.

# Remove ai_gateway/auth from https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/main/Makefile#L15
❯ make check-mypy
Installing lint dependencies...
Installing dependencies from lock file

No dependencies to install or update

Installing the current project: ai_gateway (0.1.0)
Running mypy check...
ai_gateway/auth/container.py:10: error: Need type annotation for "token_authority"  [var-annotated]
Found 1 error in 1 file (checked 110 source files)
make: *** [Makefile:100: check-mypy] Error 1

How to set up and validate locally

1. Check out to this merge request's branch.
2. Ensure the following command does not result in any warnings.

   make check-mypy

Merge request checklist

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

Contributes to #715 (closed)