Update requirements.txt

code/chapter13/helloagents-trip-planner/backend/requirements.txt

@@ -1,5 +1,5 @@
 # HelloAgents框架
-hello-agents[protocols]>=0.2.4
+hello-agents[protocols]>=0.2.4,<=0.2.9
 
 # FastAPI和相关依赖
 fastapi>=0.115.0