Ensure Duo Chat Evaluation score is not degredated with AI Gateway client

Problem

In Build a client for AI Gateway to connect duo chat (#431563 - closed), we introduced AI Gateway client for GitLab Duo chat. While this is not intended to change the business logic of the Duo Chat / Zero-shot ReACT framework, we should make sure that it doesn't degrade the chat response as we might be missing something.

This is a blocker against the global rollout on the gitlab_duo_chat_requests_to_ai_gateway feature flag.

Proposal

Run the Duo Chat Evaluation with the AI Gateway client. This is worked by @achueshev and @bcardoso- .