`dtype` of `zarr` array unexpectedly changes when `fill_value` is specified

What happened?

Opening a zarr group which contains an array of integer dtype with a fill_value results in an xarray dataset in which the array has floating-point dtype.

What did you expect to happen?

An xarray dataset in which the array has the original integer dtype.

Minimal Complete Verifiable Example

import zarr
import xarray

#Create zarr with integer dtype and fill_value
grp = zarr.open_group("test.zarr")
arr = grp.create(shape=(10,), name="array", dtype="int8", fill_value=-1)
arr.attrs['_ARRAY_DIMENSIONS'] = ['dim1']

#Open in xarray to see that the dtype is now float32
ds = xarray.open_zarr("test.zarr", consolidated=False)
ds['array'].dtype

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.

Relevant log output

No response

Anything else we need to know?

This is a result of https://github.com/pydata/xarray/issues/5475 where xarray's _FillValue has a different meaning to zarr's fill_value.

The change of dtype happens at https://github.com/pydata/xarray/blob/3c98ec7d96cc4b46664850cc7a40af2bc184fea0/xarray/coding/variables.py#L204 where xarray is trying to find a dtype where fill_value can represent "missing" data, wheras in zarr, fill_value can be any data value as its intent is to fill in missing chunks not represent missing data.

I'm not sure how best to fix this - maybe if the zarr fill value is clearly a non-missing value for the dtype then xarray should act as if it doesn't have a fill value? Happy to work on a PR if that seems to be a valid approach, although others may have thought on if that is a breaking change for some folks.

Environment

Reproduce the MVCE with zarr and xarray, then inspect xarray/coding/variables.py around line 204 and the open_zarr path. Determine how zarr's fill_value should affect dtype without treating it as missing data; done means integer arrays retain their original dtype while the existing missing-value behavior remains covered.