/// <summary>
/// A bucket without a single object: every lookup answers with a 404 and every listing is empty
/// until a test puts something in via <see cref="GiveFolder"/>, <see cref="GiveTopLevelFolders"/>
/// or <see cref="GiveObjects"/>.
/// </summary>
private static IAmazonS3 EmptyBucket()
{
    var s3 = Substitute.For<IAmazonS3>();

    // Objects written during the test become visible to later lookups, just like in a real bucket.
    var writtenObjects = new Dictionary<string, string?>(StringComparer.Ordinal);

    s3.GetObjectMetadataAsync(Arg.Any<GetObjectMetadataRequest>(), Arg.Any<CancellationToken>())
        .Returns(call =>
        {
            var key = call.Arg<GetObjectMetadataRequest>().Key;

            if (!writtenObjects.TryGetValue(key, out var owner))
                throw new AmazonS3Exception("not found") { StatusCode = System.Net.HttpStatusCode.NotFound };

            return owner is null ? new GetObjectMetadataResponse() : MetadataResponse(owner);
        });

    s3.ListObjectsV2Async(Arg.Any<ListObjectsV2Request>(), Arg.Any<CancellationToken>())
        .Returns(_ => new ListObjectsV2Response { IsTruncated = false });

    s3.CopyObjectAsync(Arg.Any<CopyObjectRequest>(), Arg.Any<CancellationToken>())
        .Returns(_ => new CopyObjectResponse());

    s3.PutObjectAsync(Arg.Any<PutObjectRequest>(), Arg.Any<CancellationToken>())
        .Returns(call =>
        {
            var request = call.Arg<PutObjectRequest>();
            writtenObjects[request.Key] = request.Metadata["efecte-org-id"];
            return new PutObjectResponse();
        });

    s3.DeleteObjectsAsync(Arg.Any<DeleteObjectsRequest>(), Arg.Any<CancellationToken>())
        .Returns(_ => new DeleteObjectsResponse());

    return s3;
}