agent: Add new panel navigation dropdown (#29539)

- [x] Ensure what appears in the dropdown is really what is accurate
- [x] Ensure keyboard navigation works:
  - [x] Switching tabs with `enter`
  - [x] Closing items from the menu item
  - [x] Opening the dropdown
  - [x] Focus assistant panel on dismiss
- [x] Add ability to close items from the dropdown menu
- [x] Persistence
- [x] Correct behavior when opening a text thread

Release Notes:

- agent: Added a navigation menu that shows the recently opened threads.
The button to see the full history view has been changed inside this
menu.

---------

Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Co-authored-by: Cole Miller <m@cole-miller.net>
Co-authored-by: Bennet Bo Fenner <bennet@zed.dev>
Co-authored-by: Cole Miller <cole@zed.dev>

crates/agent/src/context_picker.rs

@@ -267,7 +267,7 @@ impl ContextPicker {
                         context_picker.update(cx, |this, cx| this.select_entry(entry, window, cx))
                     })
             }))
-            .keep_open_on_confirm()
+            .keep_open_on_confirm(true)
         });
 
         cx.subscribe(&menu, move |_, _, _: &DismissEvent, cx| {