acp: Clear message editor after running /login (#40116)

Release Notes:

- N/A

crates/agent_ui/src/acp/thread_view.rs

@@ -1045,6 +1045,9 @@ impl AcpThreadView {
                 return;
             };
 
+            self.message_editor
+                .update(cx, |editor, cx| editor.clear(window, cx));
+
             let connection = thread.read(cx).connection().clone();
             let can_login = !connection.auth_methods().is_empty() || self.login.is_some();
             // Does the agent have a specific logout command? Prefer that in case they need to reset internal state.