assistant2: Add an indicator when a response is streaming in (#23078)

This PR adds an indicator to the Assistant message to indicate that it
is still streaming:

<img width="1310" alt="Screenshot 2025-01-13 at 2 10 33 PM"
src="https://github.com/user-attachments/assets/635ee60d-b5ea-40ac-952a-b7bfa7e04fcc"
/>

Release Notes:

- N/A