fix: remove serde flatten to avoid duplicate workflow_id, fix UTF-8 truncation panic

src/worker.rs

@@ -67,7 +67,6 @@ pub enum WorkerToServer {
     #[serde(rename = "update")]
     Update {
         workflow_id: String,
-        #[serde(flatten)]
         update: crate::sink::AgentUpdate,
     },
 }

src/ws_worker.rs

@@ -95,7 +95,8 @@ async fn handle_worker_socket(socket: WebSocket, state: Arc<WsWorkerState>) {
                             handle_worker_message(&state_clone, worker_msg).await;
                         }
                         Err(e) => {
-                            tracing::warn!("Failed to parse worker message: {} — raw: {}", e, &text[..text.len().min(200)]);
+                            let preview: String = text.chars().take(200).collect();
+                            tracing::warn!("Failed to parse worker message: {} — raw: {}", e, preview);
                         }
                     }
                 }