Created auto-complete commit message experiment

What does this MR do and why?

Added a button to the merge request edit commit message textarea to allow users to generate a commit message with AI.

Screenshots or screen recordings

How to set up and validate locally

• Make sure AI settings are enabled for the group
• Enable generate_commit_message_flag feature flag
• Visit a merge request
• Click Edit commit message

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.

#407367 (closed)