Function credential_env_vars 

pub const fn credential_env_vars(
    environment: LighterEnvironment,
) -> (&'static str, &'static str, &'static str)

Environment variable names for Lighter credentials.

Returns (api_key_index_var, api_secret_var, account_index_var). The api_key_index_var holds the per-account API key slot (0..=254), the api_secret_var holds the hex-encoded private key, and the account_index_var holds the account number assigned at registration.