Failing tests on main with Zarr-Python <3.1.3 due to ManifestStore change

[maxrjones]

#820 broke the tests when installing the minimum supported version of Zarr (see https://github.com/zarr-developers/VirtualiZarr/actions/runs/19087248344/job/54530154308). We need to fix the ManifestStore implementation to support both older versions of Zarr (which expects the supports_partial_writes method) and newer versions (where supports_partial_writes was removed from the base class).