Zarr backend isn't configured with supports_groups=True

[sjperkins]

What happened?

xarray doesn't recognise a zarr datatree because the zarr backend isn't configured with support_groups=True. This causes xarray.open_datatree to fail with the following exception:

Traceback (most recent call last):
  File "/home/simon/code/xarray/xarray/tests/test_dt_zarr.py", line 23, in <module>
    dt = xr.open_datatree(PATH)
         ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/simon/code/xarray/xarray/backends/api.py", line 1047, in open_datatree
    engine = plugins.guess_engine(filename_or_obj, must_support_groups=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/simon/code/xarray/xarray/backends/plugins.py", line 212, in guess_engine
    raise ValueError(error_msg)
ValueError: did not find a match in any of xarray's currently installed IO backends ['h5netcdf', 'scipy', 'netcdf4', 'pydap', 'zarr']. Consider explicitly selecting one of the installed engines via the ``engine`` parameter, or installing additional IO dependencies, see:
https://docs.xarray.dev/en/stable/getting-started-guide/installing.html
https://docs.xarray.dev/en/stable/user-guide/io.html

The following PR introduced the BackendEntrypoint.support_groups class variable:

• Change default netCDF engine to use h5netcdf and add netcdf_engine_order #10755

What did you expect to happen?

open_datatree should be able to guess the engine

Minimal Complete Verifiable Example

# /// script
# requires-python = ">=3.11"
# dependencies = [
#   "xarray[complete]@git+https://github.com/pydata/xarray.git@main",
# ]
# ///
#
# This script automatically imports the development branch of xarray to check for issues.
# Please delete this header if you have _not_ tested this script with `uv run`!

import xarray as xr
xr.show_versions()
# your reproducer code ...
import xarray as xr

def modify(ds):
  return ds

set1_data = modify(xr.Dataset({"a": 0, "b": 1}))
set2_data = modify(xr.Dataset({"a": ("x", [2, 3]), "b": ("x", [0.1, 0.2])}))
root_data = modify(xr.Dataset({"a": ("y", [6, 7, 8]), "set0": ("x", [9, 10])}))

root = xr.DataTree.from_dict(
{
  "/": root_data,
  "/set1": set1_data,
  "/set1/set1": None,
  "/set1/set2": None,
  "/set2": set2_data,
  "/set2/set1": None,
  "/set3": None,
})

PATH = "/tmp/datatree.zarr"
root.to_zarr(PATH, compute=True, consolidated=True, mode="w")
dt = xr.open_datatree(PATH)

Steps to reproduce

No response

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

Traceback (most recent call last):
  File "/home/simon/code/xarray/xarray/tests/test_dt_zarr.py", line 23, in <module>
    dt = xr.open_datatree(PATH)
         ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/simon/code/xarray/xarray/backends/api.py", line 1047, in open_datatree
    engine = plugins.guess_engine(filename_or_obj, must_support_groups=True)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/simon/code/xarray/xarray/backends/plugins.py", line 212, in guess_engine
    raise ValueError(error_msg)
ValueError: did not find a match in any of xarray's currently installed IO backends ['h5netcdf', 'scipy', 'netcdf4', 'pydap', 'zarr']. Consider explicitly selecting one of the installed engines via the ``engine`` parameter, or installing additional IO dependencies, see:
https://docs.xarray.dev/en/stable/getting-started-guide/installing.html
https://docs.xarray.dev/en/stable/user-guide/io.html

Anything else we need to know?

No response

Environment