fix(ui): P0 切换空线程不显示错误横幅

thread.error 在 useStream 切换 threadId 时不会自动清除，
导致切换到空新对话时残留旧线程的 error 触发红色错误横幅。
修复：activeMessages.length === 0 时抑制错误横幅，
空线程不可能有工具执行异常，无需展示该提示。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

langgraph/src/components/ChatMessages.tsx

@@ -435,8 +435,8 @@ export function ChatMessages({
           ));
         })()}
 
-      {/* Thread error banner */}
-      {!!thread.error && !thread.isLoading && (
+      {/* Thread error banner — only show when there are messages (suppress stale error on empty thread switch) */}
+      {!!thread.error && !thread.isLoading && activeMessages.length > 0 && (
         <div className="rounded-xl border border-red-300 dark:border-red-800 bg-red-50 dark:bg-red-950/30 px-4 py-4 flex flex-col gap-3 my-2">
           <div className="flex items-start gap-2.5">
             <AlertTriangle className="size-4 shrink-0 text-red-500 dark:text-red-400 mt-0.5" />