Only summarize reviews when there is more than 1 comment

What does this MR do and why?

This MR skips the creation of review summary when there is only 1 comment in the review. This is an attempt to avoid generating unnecessary summaries.

Related to #421028 (closed)

How to set up and validate locally

1. Enable automatically_summarize_mr_review.
2. Follow https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally.
3. Open a MR.
4. As a reviewer, start a new review and make more than 2 comments and submit review.
5. Confirm the presence of review summary in the author's todo list.
6. Repeat the step 4 as a reviewer and start a new review, but only 1 comment and submit review.
7. Confirm the absence of review summary in the author's todo list.

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.

Related to #421028 (closed)