chore: fix mypy errors in ai_gateway/chat/

What does this merge request do and why?

• There were 47 Errors in the subdirectory of ai_gateway/chat/ in AIGW this MR addresses all of them.
  • See linked issue for full list
• Additionally this MR also discovered and corrected a bug that was undermining the immutability of some of our PyDantic models
  • See: !2205 (23531023) for more details

How to set up and validate locally

This can be validated by running tests, linting, and using AIGW on your local environment. Additionally our CI setup is rather comprehensive for the changes made.

The commits are mostly broken up by error type so for a more scoped view on this MR that may be of assistance. I recognise that this is a longer MR. This MR was made before our latest deconstruction strategy, in future these MRs will be smaller.

If you have any questions I am here to help in the comments 😄

Merge request checklist

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

Closes #841 (closed)