fix(webui): keep late reasoning attached above the answer

Some providers only surface structured `reasoning_content` after answer
text has already streamed. The WebUI was treating those late
`reasoning_delta` frames as a fresh assistant placeholder, so the
Thinking bubble rendered below the already-visible answer.

Attach late reasoning back to the active assistant turn instead. The
bubble still renders above the message content, preserving the expected
Thinking -> answer order even when the provider protocol delivers the
reasoning post-hoc. Added a regression test for answer-first followed by
reasoning_delta/reasoning_end.

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

webui/src/tests/useNanobotStream.test.tsx

@@ -209,6 +209,35 @@ describe("useNanobotStream", () => {
     expect(result.current.messages[0].reasoningStreaming).toBe(false);
   });
 
+  it("attaches post-hoc reasoning to the same assistant turn above the answer", () => {
+    const fake = fakeClient();
+    const { result } = renderHook(() => useNanobotStream("chat-r5", EMPTY_MESSAGES), {
+      wrapper: wrap(fake.client),
+    });
+
+    act(() => {
+      fake.emit("chat-r5", {
+        event: "delta",
+        chat_id: "chat-r5",
+        text: "hi~",
+      });
+      fake.emit("chat-r5", { event: "stream_end", chat_id: "chat-r5" });
+      fake.emit("chat-r5", {
+        event: "reasoning_delta",
+        chat_id: "chat-r5",
+        text: "This reasoning arrived after the answer stream.",
+      });
+      fake.emit("chat-r5", { event: "reasoning_end", chat_id: "chat-r5" });
+    });
+
+    expect(result.current.messages).toHaveLength(1);
+    expect(result.current.messages[0].content).toBe("hi~");
+    expect(result.current.messages[0].reasoning).toBe(
+      "This reasoning arrived after the answer stream.",
+    );
+    expect(result.current.messages[0].reasoningStreaming).toBe(false);
+  });
+
   it("attaches assistant media_urls to complete messages", () => {
     const fake = fakeClient();
     const { result } = renderHook(() => useNanobotStream("chat-m", EMPTY_MESSAGES), {