feat(chat): Update trace ID handling in chat stream API

- Modified `chat_stream_api` to use `new_trace()` instead of an empty string for `trace_id`, enhancing traceability.
- Added tests to verify the correct generation and propagation of `trace_id` in responses, ensuring no empty trace IDs are sent to clients.

This update improves the tracking of chat sessions and ensures compliance with traceability standards.

app/api/chat.py

@@ -394,7 +394,7 @@ def chat_stream_api(
     """流式对话（方案 C）：与 POST "" 同守卫，逐块推送 LLM 文本。"""
     agent_type, message = _guard_request(req, request, auth)
     sid, customer_id = _prepare_turn(req, auth, agent_type)
-    trace_id = current_trace() or ""
+    trace_id = current_trace() or new_trace()
     has_disclaimer = agent_service.needs_disclaimer(agent_type)
     history = memory_service.get_recent(agent_type, sid)