Fix invalid HTML nesting in reasoning trigger during complex task rendering (#2382)

* Initial plan

* fix(frontend): avoid invalid paragraph nesting in reasoning trigger

Agent-Logs-Url: https://github.com/bytedance/deer-flow/sessions/4c9eb0c2-ff29-4629-a61c-4e33d736d918

Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>

* test(frontend): strengthen reasoning trigger DOM nesting assertion

Agent-Logs-Url: https://github.com/bytedance/deer-flow/sessions/4c9eb0c2-ff29-4629-a61c-4e33d736d918

Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: WillemJiang <219644+WillemJiang@users.noreply.github.com>

frontend/src/components/ai-elements/reasoning.tsx

@@ -123,9 +123,9 @@ const defaultGetThinkingMessage = (isStreaming: boolean, duration?: number) => {
     return <Shimmer duration={1}>Thinking...</Shimmer>;
   }
   if (duration === undefined) {
-    return <p>Thought for a few seconds</p>;
+    return <span>Thought for a few seconds</span>;
   }
-  return <p>Thought for {duration} seconds</p>;
+  return <span>Thought for {duration} seconds</span>;
 };
 
 export const ReasoningTrigger = memo(

frontend/tests/unit/core/reasoning-trigger.test.ts

@@ -0,0 +1,28 @@
+import { createElement } from "react";
+import { renderToStaticMarkup } from "react-dom/server";
+import { expect, test, vi } from "vitest";
+
+vi.mock("streamdown", () => ({
+  Streamdown: ({ children }: { children: string }) =>
+    createElement("div", null, children),
+}));
+
+import {
+  Reasoning,
+  ReasoningContent,
+  ReasoningTrigger,
+} from "@/components/ai-elements/reasoning";
+
+test("ReasoningTrigger default message uses phrasing content", () => {
+  const html = renderToStaticMarkup(
+    createElement(
+      Reasoning,
+      { isStreaming: false, defaultOpen: false },
+      createElement(ReasoningTrigger, null),
+      createElement(ReasoningContent, null, "test"),
+    ),
+  );
+
+  expect(html).toContain("Thought for a few seconds");
+  expect(html).not.toMatch(/<button\b[^>]*>[\s\S]*?<p\b/i);
+});