Function fold_function_call 

pub fn fold_function_call(
    name: &str,
    args: &[SqlExpr],
    registry: &FunctionRegistry,
) -> Option<SqlValue>

Fold a function call by recursively folding its arguments, dispatching through the shared scalar evaluator, and converting the result back to SqlValue. Only folds functions that are present in registry, so callers can distinguish “unknown function” from “known function, all args folded”.