[Gen AI] Update Panel header and small UI updates

[fisher-alice]

This PR updates AichatView so that the copy/clear buttons are rendered in the chat workspace and not the presentation view. It also updates chat workspace so that it is scrollable and assigns keys to list items to avoid warnings in dev console.

Before:

Panel headers -

After:

Panel headers -

Chat workspace is scrollable:

scrollable.workspace.mp4

Links

Noticed on staging the unintended change from #57879.

Testing story

Tested locally on /allthethings.

Deployment strategy

Follow-up work

Privacy

Security

Caching

PR Checklist:

• Tests provide adequate coverage
• Privacy and Security impacts have been assessed
• Code is well-commented
• New features are translatable or updates will not break translations
• Relevant documentation has been added or updated
• User impact is well-understood and desirable
• Pull Request is labeled appropriately
• Follow-up work items (including potential tech debt) are tracked and linked