Add "text generated by AI" message to comment summary

What does this MR do and why?

Add "generated by AI" text to Comments summary, as requested in https://gitlab.com/gitlab-org/gitlab/-/issues/407887#note_1396072461

Screenshots or screen recordings

How to set up and validate locally

1. In rails console enable the experiment fully

   Feature.enable(:openai_experimentation)
   Feature.enable(:summarize_comments)

2. On a public project, visit the issue page with comments and click Summarize comments button

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.