Reduce unused dependencies between ML/models and LLM/agents modules

This is LLM-related logic, similar to PhabricatorPatch, which lives in the same file.

I agree with you that it needs better organization. I want to do a general refactoring to reduce unused dependencies between ML/models and LLM/agents modules, which will also organize this.

Originally posted by @suhaibmujahid in https://github.com/mozilla/bugbug/pull/5293#discussion_r2395537563