Function eval_builtin_pad_simple 

pub fn eval_builtin_pad_simple(
    bf: &BuiltinFunctionCall,
    direction: PadDirection,
) -> Result<PushValue, CodegenError>

Evaluates __LEFTPAD or __RIGHTPAD builtin function

This is a simplified version that only handles literal arguments and nested builtin calls, but not constant references. For constant references, the caller should resolve them first.