i want to create a mcp server base on streamablehttp_stateless running mode(https://github.com/modelcontextprotocol/python-sdk/blob/main/examples/servers/simple-streamablehttp-stateless/mcp_simple_streamablehttp_stateless/server.py example code), but i can not find any mothed to get http request msg in the tools logic. so how can i get the http request headers msg in the tools define function ?