Function tool_fn 

pub fn tool_fn<P, Ret, F, Counterpart>(
    name: impl ToString,
    description: impl ToString,
    func: F,
    tool_future_fn: impl for<'a> Fn(&'a F, P, McpConnectionTo<Counterpart>) -> BoxFuture<'a, Result<Ret, Error>> + Send + Sync + 'static,
) -> (impl McpTool<Counterpart> + 'static, impl RunWithConnectionTo<Counterpart>)
where
    Counterpart: Role,
    P: JsonSchema + DeserializeOwned + 'static + Send,
    Ret: JsonSchema + Serialize + 'static + Send,
    F: AsyncFn(P, McpConnectionTo<Counterpart>) -> Result<Ret, Error> + Send + Sync + 'static,

Create a stateless function-backed MCP tool and its concurrent responder.