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/assistant/src/assistant_panel.rs

@@ -33,6 +33,7 @@ use settings::{Settings, update_settings_file};
 use smol::stream::StreamExt;
 
 use std::ops::Range;
+use std::path::Path;
 use std::{ops::ControlFlow, path::PathBuf, sync::Arc};
 use terminal_view::{TerminalView, terminal_panel::TerminalPanel};
 use ui::{ContextMenu, PopoverMenu, Tooltip, prelude::*};
@@ -1080,7 +1081,7 @@ impl AssistantPanel {
 
     pub fn open_saved_context(
         &mut self,
-        path: PathBuf,
+        path: Arc<Path>,
         window: &mut Window,
         cx: &mut Context<Self>,
     ) -> Task<Result<()>> {
@@ -1391,7 +1392,7 @@ impl AssistantPanelDelegate for ConcreteAssistantPanelDelegate {
     fn open_saved_context(
         &self,
         workspace: &mut Workspace,
-        path: PathBuf,
+        path: Arc<Path>,
         window: &mut Window,
         cx: &mut Context<Workspace>,
     ) -> Task<Result<()>> {