Refactor generate test file to follow upstream changes

What does this MR do and why?

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/3101

This is a follow up MR with: !1798 (merged)

Upstream modified the files we depend on, so it needs to be refactored to follow the changes.

Related upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/126724

Screenshots or screen recordings

How to set up and validate locally

• Simulate a SaaS instance
• Feature.enable(:openai_experimentation)
• Feature.enable(:ai_related_settings, namespace)
• Feature.enable(:generate_test_file_flag, user)

MR acceptance checklist

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

• I have evaluated the MR acceptance checklist for this MR.