Update diff summary on todos to be truncated

What does this MR do and why?

Truncates diff summary on merge requests todos and links to the summary on merge requests to auto-expand.

Screenshots or screen recordings

How to set up and validate locally

• Setup AI locally (https://docs.gitlab.com/ee/development/ai_features/#test-ai-features-locally)
• Enable summarize_diff_automatically
• Push new changes to merge request

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.

#423320 (closed)