Function serve 

pub async fn serve<R, W>(
    server: &McpServer,
    reader: R,
    writer: &mut W,
) -> Result<()>
where
    R: AsyncBufRead + Unpin,
    W: AsyncWrite + Unpin,

The stdio loop: newline-delimited JSON-RPC in, one response line per request out. Generic over the reader/writer so tests can run the whole handshake in-process. EOF on stdin (the client closed the session) is a clean shutdown.