Appending to a ZARR dataset removes dimension separator attribute

What happened?

When creating a ZARR store with dimension_separator="/", the to_zarr_store method removes dimension_separator attribute in the .zarray file.

What did you expect to happen?

It should retain the original storage attributes.

Minimal Complete Verifiable Example

import json

import xarray as xr
from zarr.storage import MemoryStore


store = MemoryStore(dimension_separator="/")

ds = xr.Dataset(
    {"data": xr.DataArray([0.0], dims=("x",))},
    coords={
        "x": xr.DataArray([0.0], dims=("x",)),
    },
)
ds.to_zarr(store, mode="w", consolidated=False)
assert json.loads(store["data/.zarray"].decode()).get("dimension_separator") == "/"

ds = xr.Dataset(
    {"data": xr.DataArray([1.0], dims=("x",))},
    coords={
        "x": xr.DataArray([1.0], dims=("x",)),
    },
)
ds.to_zarr(
    store,
    append_dim="x",
    consolidated=False
)
assert json.loads(store["data/.zarray"].decode()).get("dimension_separator") == "/"

MVCE confirmation

• Minimal example — the example is as focused as reasonably possible to demonstrate the underlying issue in xarray.
• Complete example — the example is self-contained, including all data and the text of any traceback.
• Verifiable example — the example copy & pastes into an IPython prompt or Binder notebook, returning the result.
• New issue — a search of GitHub Issues suggests this is not a duplicate.
• Recent environment — the issue occurs with the latest version of xarray and its dependencies.

Relevant log output

AssertionError                            Traceback (most recent call last)
File ~/xarray_repro/run.py:29
     18 ds = xr.Dataset(
     19     {"data": xr.DataArray([1.0], dims=("x",))},
     20     coords={
     21         "x": xr.DataArray([1.0], dims=("x",)),
     22     },
     23 )
     24 ds.to_zarr(
     25     store,
     26     append_dim="x",
     27     consolidated=False
     28 )
---> 29 assert json.loads(store["data/.zarray"].decode()).get("dimension_separator") == "/"

AssertionError:

Anything else we need to know?

No response

Environment

Start with the to_zarr append path and reproduce the provided MemoryStore example using append_dim="x". Inspect how the data/.zarray metadata is updated during the append, then verify that the dimension_separator attribute remains "/" after the second write.