assistant2: Adjust spacing and icons on the context picker (#23607)

Just fine-tuning spacing, icon size and color, and ensure they're are
consistent throughout.

Release Notes:

- N/A

crates/assistant2/src/context_picker/thread_context_picker.rs

@@ -209,9 +209,13 @@ pub fn render_thread_context_entry(
     });
 
     h_flex()
-        .gap_1()
+        .gap_1p5()
         .w_full()
-        .child(Icon::new(IconName::MessageCircle).size(IconSize::Small))
+        .child(
+            Icon::new(IconName::MessageCircle)
+                .size(IconSize::XSmall)
+                .color(Color::Muted),
+        )
         .child(Label::new(thread.summary.clone()))
         .child(div().w_full())
         .when(added, |el| {