Function auth_token_message 

pub fn auth_token_message(
    deadline_unix_secs: i64,
    account_index: i64,
    api_key_index: u8,
) -> String

ASCII auth-token message: "{deadline}:{account}:{api_key}".

Public for the rare caller that needs to recompute the signed preimage (e.g., to verify a token against a known public key). The message format matches the Go reference verbatim.