Function to_contract_units_decimal 

pub fn to_contract_units_decimal(_symbol: &str, size: Decimal) -> Decimal

Convert human-readable size (Decimal) to contract units (Decimal).

Arguments

• symbol - The trading symbol (e.g., “BTC-PERP”, “ETH-PERP”)
• size - Human-readable size as Decimal (e.g., 1.0 BTC)

Returns

Size in contract units as Decimal (e.g., 1.0 -> 1_000_000)