assistant2: Add link styles for thread messages (#22560)

<img width="700" alt="Screenshot 2025-01-02 at 1 52 30 AM"
src="https://github.com/user-attachments/assets/8d2308c8-cdea-421f-b9ff-7893479dba3c"
/>


Release Notes:

- N/A

crates/assistant2/src/active_thread.rs

@@ -5,7 +5,7 @@ use collections::HashMap;
 use gpui::{
     list, AbsoluteLength, AnyElement, AppContext, DefiniteLength, EdgesRefinement, Empty, Length,
     ListAlignment, ListOffset, ListState, Model, StyleRefinement, Subscription,
-    TextStyleRefinement, View, WeakView,
+    TextStyleRefinement, UnderlineStyle, View, WeakView,
 };
 use language::LanguageRegistry;
 use language_model::Role;
@@ -140,6 +140,15 @@ impl ActiveThread {
                 background_color: Some(colors.editor_foreground.opacity(0.01)),
                 ..Default::default()
             },
+            link: TextStyleRefinement {
+                background_color: Some(colors.editor_foreground.opacity(0.025)),
+                underline: Some(UnderlineStyle {
+                    color: Some(colors.text_accent.opacity(0.5)),
+                    thickness: px(1.),
+                    ..Default::default()
+                }),
+                ..Default::default()
+            },
             ..Default::default()
         };