I noticed that when Zed renders prompts in-window (Linux, or
`use_system_prompts: false`), clicking a prompt button also dismisses
the popover behind it. Native macOS dialogs don't have this problem
since the click goes to the OS dialog and the window never sees it.

For example, in the worktree picker:
1. Delete a dirty worktree.
2. Confirm "Force Delete".
3. Mouse down dismissed the picker, which aborted the confirmed
deletion.

This PR makes GPUI-rendered prompts behave the same way as native. If
someone actually wants to observe mouse downs during a prompt, the raw
`window.on_mouse_event` API still sees every event.

macOS:


https://github.com/user-attachments/assets/9f85622f-c0d1-44f2-83f9-8378d9d7d13b


Before GPUI prompt:


https://github.com/user-attachments/assets/d1845ba1-9886-4733-8cf6-ab32424416e1

After GPUI prompt:


https://github.com/user-attachments/assets/27936aca-8de5-4da8-88be-d88dbc1259d8

Release Notes:

- Fixed confirmation dialog buttons dismissing the popover underneath
them on click when using Zed-rendered prompts.