Function fold_function_call 

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

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”. A Volatile function is never folded.