Start a new conversation

What does this MR do and why?

The MR introduces the "new chat" functionality by supporting the special /reset quick action.

Screenshots or screen recordings

My_Movie_4

How to set up and validate locally

1. Enable AI features https://docs.gitlab.com/ee/development/ai_features.html#test-ai-features-locally
2. Enable the following feature flags:

   Feature.enable(:gitlab_duo)
   Feature.enable(:tanuki_bot)  
   Feature.enable(:ai_chat_history_context)

3. Open a new chat (on the navigation, in the help menu)
4. Ask questions, and create a new chat context by clicking the button

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 #412657