assistant2: Navigate context strip with keyboard (#23128)

Context pills are now focusable and intractable via the keyboard.

- <kbd>←</kbd> and <kbd>→</kbd> move the focus to the previous or next
item (wrapping if necessary)
- <kbd>↓</kbd> and <kbd>↑</kbd> move the focus vertically
- If the cursor is in the first/last row of the assistant/inline editor,
they will move the focus to the strip
- Inside the strip, they will move the focus to the pill horizontally
overlapping the most
- If already in the first/last row of the strip, they will move to the
first/last pill (like in editors)
- If the first/last pill is focused, they will move the focus back to
the editor
- <kbd>⌫</kbd>  removes the focused pill (unless it's the suggested one)
- <kbd>⏎</kbd> accepts the suggested pill if focused
  


https://github.com/user-attachments/assets/040bc71c-a3ae-4961-9886-2d5c3d290a73



Release Notes:

- N/A

crates/assistant2/src/inline_prompt_editor.rs

@@ -415,6 +415,8 @@ impl<T: 'static> PromptEditor<T> {
                     editor.move_to_end(&Default::default(), cx)
                 });
             }
+        } else {
+            cx.focus_view(&self.context_strip);
         }
     }
 
@@ -738,11 +740,18 @@ impl<T: 'static> PromptEditor<T> {
     fn handle_context_strip_event(
         &mut self,
         _context_strip: View<ContextStrip>,
-        ContextStripEvent::PickerDismissed: &ContextStripEvent,
+        event: &ContextStripEvent,
         cx: &mut ViewContext<Self>,
     ) {
-        let editor_focus_handle = self.editor.focus_handle(cx);
-        cx.focus(&editor_focus_handle);
+        match event {
+            ContextStripEvent::PickerDismissed
+            | ContextStripEvent::BlurredEmpty
+            | ContextStripEvent::BlurredUp => {
+                let editor_focus_handle = self.editor.focus_handle(cx);
+                cx.focus(&editor_focus_handle);
+            }
+            ContextStripEvent::BlurredDown => {}
+        }
     }
 }
 
@@ -826,7 +835,6 @@ impl PromptEditor<BufferCodegen> {
                 context_store.clone(),
                 workspace.clone(),
                 thread_store.clone(),
-                prompt_editor.focus_handle(cx),
                 context_picker_menu_handle.clone(),
                 SuggestContextKind::Thread,
                 cx,
@@ -978,7 +986,6 @@ impl PromptEditor<TerminalCodegen> {
                 context_store.clone(),
                 workspace.clone(),
                 thread_store.clone(),
-                prompt_editor.focus_handle(cx),
                 context_picker_menu_handle.clone(),
                 SuggestContextKind::Thread,
                 cx,