Add ability to dismiss summary

What does this MR do and why?

Allow dismissing AI dicussion summary

Screenshots or screen recordings

How to set up and validate locally

1. Set up discussion summary AI feature
   1. Configure GCP Vertex access
   2. Enable :summarize_comments feature flag
2. Click 'View summary' on an issue page, ideally one where the issue has comments
3. Use dropdown to copy text to clipboard & remove summary
4. Click View summary again

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 #414763 (closed)