Generate a test case for ManifestStore with scalar arrays

This issue is to track the TODO in https://github.com/zarr-developers/VirtualiZarr/pull/523#discussion_r2023159603. Zarr will use the `<array>/c` key for accessing scalar arrays, rather than `<array>/c/0` as one might expect. Currently, we will raise a NotImplementedError but we should generate a test case and properly support scalar arrays.