Function validate_percentile_expr 

pub(crate) fn validate_percentile_expr(
    expr: &Arc<dyn PhysicalExpr>,
    fn_name: &str,
) -> Result<f64>

Validates that a percentile expression is a literal float value between 0.0 and 1.0.

Used by both percentile_cont and approx_percentile_cont to validate their percentile parameters.