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/active_thread.rs

@@ -709,7 +709,7 @@ fn open_markdown_link(
             if let Some(panel) = workspace.panel::<AssistantPanel>(cx) {
                 panel.update(cx, |panel, cx| {
                     panel
-                        .open_thread(&thread_id, window, cx)
+                        .open_thread_by_id(&thread_id, window, cx)
                         .detach_and_log_err(cx)
                 });
             }
@@ -3275,7 +3275,7 @@ pub(crate) fn open_context(
                 panel.update(cx, |panel, cx| {
                     let thread_id = thread_context.thread.read(cx).id().clone();
                     panel
-                        .open_thread(&thread_id, window, cx)
+                        .open_thread_by_id(&thread_id, window, cx)
                         .detach_and_log_err(cx)
                 });
             }