assistant2: Add "running" status feedback in the disclosure (#26786)

Just a tiny bit of polish here, so that if the user expands the
disclosure, an equivalent loading state is at the response container.

<img
src="https://github.com/user-attachments/assets/a2ecb7f4-c9ea-4a14-8a60-9f7f2983a1a1"
width="600px" />

Release Notes:

- N/A