Enable LLM debug logging

What does this MR do and why?

This enables LLM logging for orchestrated tests using the AI Gateway, to help us better triage AI test failures

Currently the LLM log is empty, e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/6879663342/artifacts/browse/gitlab-qa-run-2024-05-17-14-19-57-618df2fc/gitlab/logs/gitlab-rails/

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Commits include Changelog: trailer
• I have evaluated the MR acceptance checklist for this MR.