fix(webui): keep reasoning scoped to the current user turn

The post-hoc reasoning fix allowed late reasoning frames to attach back to
the nearest assistant message, but the scan crossed a newer user message.
That made the next turn's Thinking bubble render above the previous
assistant reply.

Treat the latest user message as a hard boundary: reasoning after it must
start a new assistant placeholder and can no longer attach to earlier
assistant turns. Add a regression covering previous assistant -> new user
-> reasoning_delta.

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

webui/src/hooks/useNanobotStream.ts

@@ -30,6 +30,9 @@ interface StreamBuffer {
 function attachReasoningChunk(prev: UIMessage[], chunk: string): UIMessage[] {
   for (let i = prev.length - 1; i >= 0; i -= 1) {
     const candidate = prev[i];
+    // A user turn is a hard boundary: reasoning after it belongs to the new
+    // assistant turn, never to an earlier assistant reply.
+    if (candidate.role === "user") break;
     if (candidate.role !== "assistant" || candidate.kind === "trace") continue;
     const hasAnswer = candidate.content.length > 0;
     if (