assistant: Tweak tab bar layout (#15901)

- Add "New Context" button next to the hamburger
- Add "History"
- Allow Pane tab rendering callback to return items for both left and
right side of the tab bar.


![image](https://github.com/user-attachments/assets/d5aa599d-c9e4-4f26-ad66-ffc290c53c29)


Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -253,6 +253,7 @@ pub enum IconName {
     Tab,
     Terminal,
     TextCursor,
+    TextSearch,
     Trash,
     TriangleRight,
     Update,
@@ -414,6 +415,7 @@ impl IconName {
             IconName::Tab => "icons/tab.svg",
             IconName::Terminal => "icons/terminal.svg",
             IconName::TextCursor => "icons/text-cursor.svg",
+            IconName::TextSearch => "icons/text-search.svg",
             IconName::Trash => "icons/trash.svg",
             IconName::TriangleRight => "icons/triangle_right.svg",
             IconName::Update => "icons/update.svg",