assistant2: Add check icon for included context (#22774)

Quick follow-up to: https://github.com/zed-industries/zed/pull/22712 —
just to make it more visually easier to understand.

<img width="800" alt="Screenshot 2025-01-07 at 11 48 06 AM"
src="https://github.com/user-attachments/assets/92f0523b-eb85-4929-a825-2e1e524b3ad7"
/>

Release Notes:

- N/A

crates/assistant2/src/context_picker/thread_context_picker.rs

@@ -209,7 +209,16 @@ impl PickerDelegate for ThreadContextPickerDelegate {
                 .toggle_state(selected)
                 .child(Label::new(thread.summary.clone()))
                 .when(added, |el| {
-                    el.end_slot(Label::new("Added").size(LabelSize::XSmall))
+                    el.end_slot(
+                        h_flex()
+                            .gap_1()
+                            .child(
+                                Icon::new(IconName::Check)
+                                    .size(IconSize::Small)
+                                    .color(Color::Success),
+                            )
+                            .child(Label::new("Added").size(LabelSize::Small)),
+                    )
                 }),
         )
     }