fix(P0): 恢复 handleNewThread 立即创建线程，移除 ensureThread 懒创建

根因：ensureThread 异步 setCurrentThreadId 后 React 状态未同步刷新，
useStream 仍绑定 undefined threadId，thread.submit() 提交到错误线程。

修复：
- handleNewThread 恢复为 await client.threads.create()（立即创建）
- doSubmit 直接使用 currentThreadId（已同步可用）
- 移除 ensureThread、skipNextSwitchRef 等懒创建相关逻辑
- 空线程问题后续通过侧边栏过滤解决

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

langgraph/src/hooks/useConversation.ts

@@ -17,7 +17,7 @@ export function useConversation(
   const [resetLoading, setResetLoading] = useState(false);
   const [threadLoadFailed, setThreadLoadFailed] = useState(false);
 
-  const handleNewThread = useCallback(() => {
+  const handleNewThread = useCallback(async () => {
     if (currentThreadId) {
       try { localStorage.setItem(`draft_${currentThreadId}`, input); } catch { /* ignore */ }
     }
@@ -25,9 +25,15 @@ export function useConversation(
     setHistoricalMessages([]);
     setHistoricalUi([]);
     setThreadLoadFailed(false);
+    try {
+      const t = await client.threads.create();
+      setThreads((prev) => [t as ThreadItem, ...prev]);
+      setCurrentThreadId(t.thread_id);
+    } catch {
       setCurrentThreadId(null);
+    }
     setSidebarOpen(false);
-  }, [currentThreadId, input, setInput]);
+  }, [currentThreadId, input, client, setInput]);
 
   const handleSelectThread = useCallback(async (threadId: string) => {
     if (currentThreadId) {

langgraph/src/main.tsx

@@ -43,7 +43,7 @@ function App() {
     historicalMessages, historicalUi,
     resetLoading, threadLoadFailed,
     handleNewThread, handleSelectThread, handleDeleteThread, handleResetThread,
-    applyThreadTitle, ensureThread,
+    applyThreadTitle,
   } = useConversation(client, input, setInput);
 
   // Canvas panel state
@@ -63,8 +63,6 @@ function App() {
   type ConcurrentStatus = "idle" | "generating" | "stopping" | "cancelling";
   const [concurrentStatus, setConcurrentStatus] = useState<ConcurrentStatus>("idle");
   const wasStoppedRef = useRef(false);
-  // Skip the switchThread useEffect when doSubmit already called it manually
-  const skipNextSwitchRef = useRef(false);
 
   interface SubmitPayload {
     text: string;
@@ -97,13 +95,8 @@ function App() {
     return () => window.removeEventListener("open-canvas", handler);
   }, []);
 
-  // Clear error and stale stream state when thread switches.
-  // Skip when doSubmit set the flag (new thread created during submit — don't clear mid-stream).
+  // Clear error and stale stream state when thread switches
   useEffect(() => {
-    if (skipNextSwitchRef.current) {
-      skipNextSwitchRef.current = false;
-      return;
-    }
     thread.switchThread(currentThreadId);
   }, [currentThreadId]); // eslint-disable-line react-hooks/exhaustive-deps
 
@@ -211,21 +204,13 @@ function App() {
     setInput("");
     setSourceLabel(null);
 
-    // Lazy thread creation: create a real thread only when the user first sends a message
-    const activeThreadId = await ensureThread();
-    if (!activeThreadId) {
+    if (!currentThreadId) {
       setConcurrentStatus("idle");
       return;
     }
-    // For a newly created thread, currentThreadId just changed via setCurrentThreadId
-    // but React state hasn't flushed yet. Skip the useEffect switchThread call to avoid
-    // clearing thread.messages mid-stream. useStream picks up the new threadId via prop.
-    if (!currentThreadId) {
-      skipNextSwitchRef.current = true;
-    }
 
-    try { localStorage.removeItem(`draft_${activeThreadId}`); } catch { /* ignore */ }
-    updateThreadLastActive(activeThreadId);
+    try { localStorage.removeItem(`draft_${currentThreadId}`); } catch { /* ignore */ }
+    updateThreadLastActive(currentThreadId);
 
     const enabledTools =
       tools.size > 0
@@ -269,9 +254,9 @@ function App() {
     // Scroll to bottom after submitting so user message stays visible
     setTimeout(() => bottomRef.current?.scrollIntoView({ behavior: "smooth" }), 100);
 
-    if (activeThreadId && text) {
-      const existingTitle = threads.find((t) => t.thread_id === activeThreadId)?.metadata?.title as string | undefined;
-      applyThreadTitle(activeThreadId, text, activeMessages.length, existingTitle);
+    if (currentThreadId && text) {
+      const existingTitle = threads.find((t) => t.thread_id === currentThreadId)?.metadata?.title as string | undefined;
+      applyThreadTitle(currentThreadId, text, activeMessages.length, existingTitle);
     }
   }