Function server_first_message 

pub fn server_first_message(
    username: &str,
    client_nonce_b64: &str,
    credentials: &ScramCredentials,
) -> (ScramHandshake, String)

Server-side: Create the server-first-message data and handshake state.

username is taken from the HELLO phase. client_nonce_b64 is the raw client nonce (as returned by client_first_message). credentials are the pre-computed SCRAM credentials for this user.

Returns (handshake_state, server_first_data_base64).