Writing multiscale element to disk fails when configuring Dask to uses "processes"

When configuring dask to use "processes" instead of "threads", writing a multiscale element to disk fails. I get the error message:

File ~/VIB/harpy/.venv_harpy/lib/python3.12/site-packages/spatialdata/_core/spatialdata.py:1177, in SpatialData.write(self, file_path, overwrite, consolidate_metadata, update_sdata_path, sdata_formats)
   1174 store.close()
   1176 for element_type, element_name, element in self.gen_elements():
-> 1177     self._write_element(
   1178         element=element,
   1179         zarr_container_path=file_path,
...
--> 120 if group.metadata.zarr_format == 3 and len(multiscales := group.metadata.attributes["ome"]["multiscales"]) != 1:
    121     len_scales = len(multiscales)
    122     raise ValueError(f"The length of multiscales metadata should be 1, found the length to be {len_scales}")

KeyError: 'ome'

Minimal example to reproduce:

import os
from spatialdata.models import Image2DModel

from spatialdata.datasets import blobs

import dask

with dask.config.set(scheduler='processes'):

    sdata = blobs()
    sdata=sdata.subset( element_names=[ "blobs_image" ] )
    sdata.write(os.path.join( os.environ.get("TMPDIR"), "sdata.zarr" ), overwrite=True   ) # this works
    print( "done" )

    sdata = blobs()
    sdata=sdata.subset( element_names=[ "blobs_multiscale_image" ] )
    sdata.write(os.path.join( os.environ.get("TMPDIR"), "sdata.zarr" ), overwrite=True   )
# this fails with
#--> 120 if group.metadata.zarr_format == 3 and len(multiscales := group.metadata.attributes["ome"]["multiscales"]) != 1:
#    121     len_scales = len(multiscales)
#    122     raise ValueError(f"The length of multiscales metadata should be 1, found the length to be {len_scales}")

#KeyError: 'ome'

with dask.config.set(scheduler='threads'):

    sdata = blobs()
    sdata=sdata.subset( element_names=[ "blobs_image" ] )
    sdata.write(os.path.join( os.environ.get("TMPDIR"), "sdata.zarr" ), overwrite=True   )
    print( "done" )

    sdata = blobs()
    sdata=sdata.subset( element_names=[ "blobs_multiscale_image" ] )
    sdata.write(os.path.join( os.environ.get("TMPDIR"), "sdata.zarr" ), overwrite=True   )
    print("done")

I tested this on MacOS and CentOS, and got the same error message.

Working with spatialdata==0.6.0, spatial_image==1.2.3 and multiscale_spatial_image==2.0.3.

I also noticed, for images and labes, that the layer in the dask graph that starts with "from-zarr-" is no longer materialized when the spatialdata object is backed by a zarr store (zarr>=3). Probably this is unrelated to the current issue, but I found this a bit strange, since this was not the case in earlier versions of spatialdata(<0.5.0), see https://github.com/saeyslab/harpy/blob/609d639c7578a4c64c3eede4c974d4e90f982910/src/harpy/_tests/test_image/test_manager.py#L47

Reproduce the failure with the provided blobs() examples and Dask's processes scheduler, then inspect spatialdata/_core/spatialdata.py around SpatialData.write and the reported metadata check near line 120. Compare the processes and threads execution paths for multiscale images; done means the multiscale element writes successfully under processes without the KeyError, with regression coverage for the reproduced case.