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/assistant_panel.rs

@@ -300,11 +300,12 @@ impl AssistantPanel {
     fn render_toolbar(&self, cx: &mut ViewContext<Self>) -> impl IntoElement {
         let focus_handle = self.focus_handle(cx);
 
-        let title = if self.thread.read(cx).is_empty() {
-            SharedString::from("New Thread")
+        let thread = self.thread.read(cx);
+
+        let title = if thread.is_empty() {
+            thread.summary_or_default(cx)
         } else {
-            self.thread
-                .read(cx)
+            thread
                 .summary(cx)
                 .unwrap_or_else(|| SharedString::from("Loading Summary…"))
         };