acp thread view: Always use editors for user messages (#36256)

This means the cursor will be at the position you clicked:


https://github.com/user-attachments/assets/0693950d-7513-4d90-88e2-55817df7213a


Release Notes:

- N/A

crates/agent_ui/src/agent_panel.rs

@@ -818,12 +818,10 @@ impl AgentPanel {
             ActiveView::Thread { thread, .. } => {
                 thread.update(cx, |thread, cx| thread.cancel_last_completion(window, cx));
             }
-            ActiveView::ExternalAgentThread { thread_view, .. } => {
-                thread_view.update(cx, |thread_element, cx| {
-                    thread_element.cancel_generation(cx)
-                });
-            }
-            ActiveView::TextThread { .. } | ActiveView::History | ActiveView::Configuration => {}
+            ActiveView::ExternalAgentThread { .. }
+            | ActiveView::TextThread { .. }
+            | ActiveView::History
+            | ActiveView::Configuration => {}
         }
     }