Provide ChatGLM support for Summarize MR Diff

Related issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/3170

Dependent MRs:

• Upstream MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/124688
• JH MR: !1785 (merged)

What does this MR do and why?

Provide ChatGLM support for Summarize MR Diff.

Screenshots or screen recordings

MR diff

Quick action

Comment from GitLab LLM Bot

How to set up and validate locally

• Simulate a SaaS instance
• Create a Project and MRs under a Ultimate Group
• Feature.enable(:openai_experimentation)
• Feature.enable(:ai_related_settings, namespace)
• namespace.update!(experiment_features_enabled: true)
• Feature.enable(:summarize_diff_quick_action, user)

Note: Do not enable FF summarize_diff_automatically.

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.