agent: Support images via @file and the file context picker (#29596)

Release Notes:

- agent: Add support for @mentioning images
- agent: Add support for including images via file context picker

---------

Co-authored-by: Oleksiy Syvokon <oleksiy.syvokon@gmail.com>

crates/agent/src/message_editor.rs

@@ -375,7 +375,7 @@ impl MessageEditor {
 
         self.context_store.update(cx, |store, cx| {
             for image in images {
-                store.add_image(Arc::new(image), cx);
+                store.add_image_instance(Arc::new(image), cx);
             }
         });
     }