Derive Macro ToUpdateModel 

#[derive(ToUpdateModel)]
{
    // Attributes available to this derive:
    #[crudcrate]
    #[active_model]
}

Generates <Name>Update struct with fields not excluded by exclude(update). All fields are Option<Option<T>> to support partial updates and explicit null. Implements MergeIntoActiveModel trait with on_update expression handling.