Function two_body_total_momentum 

pub fn two_body_total_momentum(
    mass_a: f64,
    velocity_a: f64,
    mass_b: f64,
    velocity_b: f64,
) -> Option<f64>

Computes the total momentum of two moving bodies using p_total = m1v1 + m2v2.

Returns None when either mass is negative, when any input is not finite, or when the total momentum is not finite.