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

backend/app/agent/agent_service.py

@@ -7,7 +7,7 @@ import json
 import asyncio
 
 # 本地模块
-from app.main_graph.utils.subgraph_builder import build_react_main_graph
+from app.main_graph.utils.main_graph_builder import build_react_main_graph
 from app.main_graph.tools.graph_tools import AVAILABLE_TOOLS, TOOLS_BY_NAME
 from app.main_graph.config import set_stream_writer
 from ..model_services.chat_services import get_all_chat_services, LocalVLLMChatProvider