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_context_editor/src/context_editor.rs

@@ -48,7 +48,14 @@ use project::{Project, Worktree};
 use rope::Point;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsStore, update_settings_file};
-use std::{any::TypeId, cmp, ops::Range, path::PathBuf, sync::Arc, time::Duration};
+use std::{
+    any::TypeId,
+    cmp,
+    ops::Range,
+    path::{Path, PathBuf},
+    sync::Arc,
+    time::Duration,
+};
 use text::SelectionGoal;
 use ui::{
     ButtonLike, Disclosure, ElevationIndex, KeyBinding, PopoverMenuHandle, TintColor, Tooltip,
@@ -139,7 +146,7 @@ pub trait AssistantPanelDelegate {
     fn open_saved_context(
         &self,
         workspace: &mut Workspace,
-        path: PathBuf,
+        path: Arc<Path>,
         window: &mut Window,
         cx: &mut Context<Workspace>,
     ) -> Task<Result<()>>;