refactor: 重命名文件更清晰，调整最大推理步数为10

- subgraph_builder.py → main_graph_builder.py
- service.py → agent_service.py
- 调整 max_steps 从 40 → 10
- 更新所有相关导入

test/test_backend.py

@@ -19,7 +19,7 @@ load_dotenv()
 
 from backend.app.config import DB_URI
 from langgraph.checkpoint.postgres.aio import AsyncPostgresSaver
-from backend.app.agent.service import AIAgentService
+from backend.app.agent.agent_service import AIAgentService
 from backend.app.agent.history import ThreadHistoryService
 from backend.app.logger import info, warning, error