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/active_thread.rs

@@ -76,6 +76,10 @@ impl ActiveThread {
         self.thread.read(cx).summary()
     }
 
+    pub fn summary_or_default(&self, cx: &AppContext) -> SharedString {
+        self.thread.read(cx).summary_or_default()
+    }
+
     pub fn last_error(&self) -> Option<ThreadError> {
         self.last_error.clone()
     }