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/assistant.rs

@@ -45,7 +45,13 @@ actions!(
         OpenHistory,
         Chat,
         CycleNextInlineAssist,
-        CyclePreviousInlineAssist
+        CyclePreviousInlineAssist,
+        FocusUp,
+        FocusDown,
+        FocusLeft,
+        FocusRight,
+        RemoveFocusedContext,
+        AcceptSuggestedContext
     ]
 );