Function diff_preview_cancel 

pub async fn diff_preview_cancel(
    diff_id: DiffId,
    store: State<'_, DiffStoreHandle>,
) -> Result<(), AppError>

Cancel a pending diff record, voiding any future confirm attempts.

After cancellation, object_set_storage_class (or any other command that calls DiffStore::consume) will receive None and must return AppError::Validation { hint: "Diff was cancelled or expired" }.

Returns AppError::NotFound when the diff_id does not exist.