Function resolve_wallet_address 

pub fn resolve_wallet_address(
    wallet_address: Option<String>,
    is_testnet: bool,
) -> Option<String>

Resolves wallet address from config value or environment variable.

Priority:

1. If wallet_address is Some, use it directly.
2. Otherwise, try to read from environment variable.

Environment variables:

• Mainnet: DYDX_WALLET_ADDRESS
• Testnet: DYDX_TESTNET_WALLET_ADDRESS

Returns None if neither config nor env var provides a wallet address.