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/thread_store.rs

@@ -52,6 +52,14 @@ impl ThreadStore {
         })
     }
 
+    /// Returns the number of non-empty threads.
+    pub fn non_empty_len(&self, cx: &AppContext) -> usize {
+        self.threads
+            .iter()
+            .filter(|thread| !thread.read(cx).is_empty())
+            .count()
+    }
+
     pub fn threads(&self, cx: &ModelContext<Self>) -> Vec<Model<Thread>> {
         let mut threads = self
             .threads