assistant2: Sketch in sending file context to model (#21829)

This PR sketches in support for sending file context attached to a
message to the model.

Right now the context is just mocked.

<img width="1159" alt="Screenshot 2024-12-10 at 4 18 41 PM"
src="https://github.com/user-attachments/assets/3ee4e86a-7893-42dc-98f9-982aa202d310">

<img width="1159" alt="Screenshot 2024-12-10 at 4 18 53 PM"
src="https://github.com/user-attachments/assets/8a3c2dd7-a466-4dbf-83ec-1c7d969c1a4b">

Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -15,6 +15,7 @@ use ui::prelude::*;
 use workspace::Workspace;
 
 use crate::thread::{MessageId, Thread, ThreadError, ThreadEvent};
+use crate::ui::ContextPill;
 
 pub struct ActiveThread {
     workspace: WeakView<Workspace>,
@@ -202,6 +203,8 @@ impl ActiveThread {
             return Empty.into_any();
         };
 
+        let context = self.thread.read(cx).context_for_message(message_id);
+
         let (role_icon, role_name) = match message.role {
             Role::User => (IconName::Person, "You"),
             Role::Assistant => (IconName::ZedAssistant, "Assistant"),
@@ -229,7 +232,16 @@ impl ActiveThread {
                                     .child(Label::new(role_name).size(LabelSize::Small)),
                             ),
                     )
-                    .child(v_flex().p_1p5().text_ui(cx).child(markdown.clone())),
+                    .child(v_flex().p_1p5().text_ui(cx).child(markdown.clone()))
+                    .when_some(context, |parent, context| {
+                        parent.child(
+                            h_flex().flex_wrap().gap_2().p_1p5().children(
+                                context
+                                    .iter()
+                                    .map(|context| ContextPill::new(context.clone())),
+                            ),
+                        )
+                    }),
             )
             .into_any()
     }