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/ui/context_pill.rs

@@ -0,0 +1,25 @@
+use ui::prelude::*;
+
+use crate::context::Context;
+
+#[derive(IntoElement)]
+pub struct ContextPill {
+    context: Context,
+}
+
+impl ContextPill {
+    pub fn new(context: Context) -> Self {
+        Self { context }
+    }
+}
+
+impl RenderOnce for ContextPill {
+    fn render(self, cx: &mut WindowContext) -> impl IntoElement {
+        div()
+            .px_1()
+            .border_1()
+            .border_color(cx.theme().colors().border)
+            .rounded_md()
+            .child(Label::new(self.context.name.clone()).size(LabelSize::Small))
+    }
+}