toInput abstract method

Creates a corresponding input object for this model.

The returned input object can be used for insert, update, and upsert operations with all fields pre-filled with the values from this model.

Subclasses must implement this method to return their specific input type.

Example:

@override
BookInput toInput() {
  return BookInput(
    id: id,
    title: title,
    // other fields...
  );
}