assistant2: Suggest recent files and threads as context (#22959)

The context picker will now display up to 6 recent files/threads to add
as a context:

<img
src="https://github.com/user-attachments/assets/80c87bf9-70ad-4e81-ba24-7a624378b991"
width=400>



Note: We decided to use a `ContextMenu` instead of `Picker` for the
initial one since the latter didn't quite fit the design for the
"Recent" section.

Release Notes:

- N/A

---------

Co-authored-by: Danilo <danilo@zed.dev>
Co-authored-by: Piotr <piotr@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>

crates/assistant2/src/thread_history.rs

@@ -100,12 +100,8 @@ impl PastThread {
 impl RenderOnce for PastThread {
     fn render(self, cx: &mut WindowContext) -> impl IntoElement {
         let (id, summary) = {
-            const DEFAULT_SUMMARY: SharedString = SharedString::new_static("New Thread");
             let thread = self.thread.read(cx);
-            (
-                thread.id().clone(),
-                thread.summary().unwrap_or(DEFAULT_SUMMARY),
-            )
+            (thread.id().clone(), thread.summary_or_default())
         };
 
         let thread_timestamp = time_format::format_localized_timestamp(