Closes #39532

## Summary

Move the long-running agent "generating" spinner into a dedicated
`GeneratingSpinner` view.

Previously, the generating state rendered `SpinnerLabel` inline as part
of the full thread view. This change keeps the same spinner styling, but
isolates it into its own small view so the animation no longer needs to
live directly inside the larger thread UI subtree while the agent is
running.

before:

<img width="398" height="448" alt="image"
src="https://github.com/user-attachments/assets/8a56c394-f7a6-4835-b877-fbc1500d2d03"
/>


after:

<img width="524" height="597" alt="image"
src="https://github.com/user-attachments/assets/bd2e5804-dba8-49c0-a421-7dc5ba600348"
/>


## Testing

- `cargo run -j 4 ~/Downloads/WorkSpace/zed/`
- `cargo check -p agent_ui`

Release Notes:

- Fixed high GPU usage from the agent panel's generating spinner while
an agent is running.

---------

Co-authored-by: Anthony Eid <anthony@zed.dev>