assistant2: Thread history keyboard navigation (#23145)

Open and delete threads via keyboard:


https://github.com/user-attachments/assets/79b402ad-a49d-4c52-9d46-28a7bf32ff1f



Note: this doesn't include navigation in the "recent threads" section of
the empty state

Release Notes:

- N/A

crates/assistant2/src/assistant.rs

@@ -40,6 +40,7 @@ actions!(
         ToggleModelSelector,
         RemoveAllContext,
         OpenHistory,
+        RemoveSelectedThread,
         Chat,
         ChatMode,
         CycleNextInlineAssist,