fix(chat): 阻止同会话并发 Agent Run 导致消息错乱

前端 submit 改用 chatStateRef 同步拦截连发；后端 createRun 检测同 session 活跃任务并返回 409，避免多条回复交错落库。

Co-authored-by: Cursor <cursoragent@cursor.com>

agent-run-routes.test.mjs

@@ -464,6 +464,43 @@ test('POST /agent/runs rejects a session the user does not own', async () => {
   assert.deepEqual(res.body, { message: '无权访问该会话' });
 });
 
+test('POST /agent/runs returns 409 when session already has active run', async () => {
+  const handler = createPostAgentRunsHandler({
+    userAuth: {
+      async ownsSession() {
+        return true;
+      },
+    },
+    agentRunGateway: {
+      async createRun() {
+        const err = new Error('该会话有正在处理的任务，请等待完成后再发送');
+        err.code = 'SESSION_RUN_CONFLICT';
+        err.status = 409;
+        throw err;
+      },
+    },
+  });
+  const res = createResponseRecorder();
+
+  await handler(
+    {
+      currentUser: { id: 'user-1' },
+      body: {
+        session_id: 'session-busy',
+        request_id: 'req-busy',
+        user_message: { role: 'user', content: [{ type: 'text', text: 'hello' }] },
+      },
+    },
+    res,
+  );
+
+  assert.equal(res.statusCode, 409);
+  assert.deepEqual(res.body, {
+    message: '该会话有正在处理的任务，请等待完成后再发送',
+    code: 'SESSION_RUN_CONFLICT',
+  });
+});
+
 test('POST /agent/runs validates ownership through sessionAccess when provided', async () => {
   const validated = [];
   const handler = createPostAgentRunsHandler({