Function calculate_fixed_risk_position_size 

pub fn calculate_fixed_risk_position_size(
    instrument: &InstrumentAny,
    entry: Price,
    stop_loss: Price,
    equity: Money,
    risk: Decimal,
    commission_rate: Decimal,
    exchange_rate: Decimal,
    hard_limit: Option<Decimal>,
    unit_batch_size: Decimal,
    units: usize,
) -> CorrectnessResult<Quantity>

Calculates the position size based on fixed risk parameters.

Errors

Returns an error if an input is invalid, decimal arithmetic overflows, or the final size cannot be represented as a [Quantity].