Integrate calculate_acceptance_threshold into spec

[SeanMcOwen]

Probably just as a policy option within ride pricing

def calculate_acceptance_threshold(self, ride_metadata):
    threshold = (
        ride_metadata['cost_usd'] * self.preferences['cost_weight'] +
        ride_metadata['distance_km'] * self.preferences['distance_weight'] +
        ride_metadata['duration_min'] * self.preferences['duration_weight'] +
        ride_metadata['driver_rating'] * self.preferences['driver_rating_weight'] +
        ride_metadata['rider_rating'] * self.preferences['rider_rating_weight']
    )
    return threshold