assistant2: Rework @mentions (#26983)

https://github.com/user-attachments/assets/167f753f-2775-4d31-bfef-55565e61e4bc

Release Notes:

- N/A

crates/ui/src/components/icon.rs

@@ -374,7 +374,7 @@ enum IconSource {
 impl IconSource {
     fn from_path(path: impl Into<SharedString>) -> Self {
         let path = path.into();
-        if path.starts_with("icons/file_icons") {
+        if path.starts_with("icons/") {
             Self::Svg(path)
         } else {
             Self::Image(Arc::from(PathBuf::from(path.as_ref())))