Resolve "Auto-generate change summaries"

What does this MR do and why?

We would like to generate the summary automatically for merge request diffs.

To Test:

1. Follow https://docs.gitlab.com/ee/development/ai_features.html#features for configuring local environment to be able to work with Open api.
2. Follow testing guide for AI features: https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally.
3. Turn on feature flag: summarize_diff_automatically
4. Ensure your license has summarize_mr_changes available
5. Push a new commit
6. Check in rails console if the summary was generated (MergeRequest.find(ID).merge_request_diff.merge_request_diff_llm_summary)

Related to #408434 (closed)