Function resolve_credentials 

pub fn resolve_credentials(
    config_api_key: Option<String>,
    config_api_secret: Option<String>,
    environment: BinanceEnvironment,
    product_type: BinanceProductType,
) -> Result<(String, String)>

Resolves API credentials from config or environment variables.

Checks standard environment variables first, then falls back to deprecated *_ED25519_* variables with a deprecation warning.

For live environments:

• Deprecated: BINANCE_ED25519_API_KEY / BINANCE_ED25519_API_SECRET
• Standard: BINANCE_API_KEY / BINANCE_API_SECRET

For testnet environments (Spot):

• Deprecated: BINANCE_TESTNET_ED25519_API_KEY / BINANCE_TESTNET_ED25519_API_SECRET
• Standard: BINANCE_TESTNET_API_KEY / BINANCE_TESTNET_API_SECRET

For testnet environments (Futures):

• Deprecated: BINANCE_FUTURES_TESTNET_ED25519_API_KEY / BINANCE_FUTURES_TESTNET_ED25519_API_SECRET
• Standard: BINANCE_FUTURES_TESTNET_API_KEY / BINANCE_FUTURES_TESTNET_API_SECRET

Errors

Returns an error if credentials cannot be resolved from config or environment.