Function periapsis_from_semi_major_axis_eccentricity 

pub fn periapsis_from_semi_major_axis_eccentricity(
    semi_major_axis: f64,
    eccentricity: f64,
) -> Option<f64>

Computes periapsis radius from semi-major axis and eccentricity.

Formula: r_p = a * (1 - e)

Returns None when semi_major_axis is less than or equal to zero, when eccentricity is outside [0.0, 1.0), or when the input or result is not finite.