assistant2: Combine history views into one (#25293)

This PR combines the two history views in Assistant2 into one.

<img width="1309" alt="Screenshot 2025-02-20 at 5 34 37 PM"
src="https://github.com/user-attachments/assets/fbb08542-58b5-4930-8a20-254234e335fa"
/>

<img width="1309" alt="Screenshot 2025-02-20 at 5 34 41 PM"
src="https://github.com/user-attachments/assets/1174849e-edad-4e02-8bf3-bb92aafba4f8"
/>


Release Notes:

- N/A

crates/assistant_context_editor/src/context_store.rs

@@ -350,6 +350,12 @@ impl ContextStore {
         }
     }
 
+    pub fn contexts(&self) -> Vec<SavedContextMetadata> {
+        let mut contexts = self.contexts_metadata.iter().cloned().collect::<Vec<_>>();
+        contexts.sort_unstable_by_key(|thread| std::cmp::Reverse(thread.mtime));
+        contexts
+    }
+
     pub fn create(&mut self, cx: &mut Context<Self>) -> Entity<AssistantContext> {
         let context = cx.new(|cx| {
             AssistantContext::local(