Open existing groups when writing a DataTree with region/append_dim

virtual_datatree_to_icechunk unconditionally created each node's group
with Group.from_store, so forwarding region or append_dim (which require
the group and its arrays to already exist) always failed with
ContainsGroupError. Mirror the branch virtual_dataset_to_icechunk
already had: open the existing group for region/append_dim, create it
otherwise.

Removes the now-passing xfail on test_write_datatree_region, whose
stated reason (an xarray region limitation) was a misdiagnosis of this
ContainsGroupError.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: aaronspring

docs/about/releases.md

@@ -9,6 +9,9 @@
 
 ### Bug fixes
 
+- Writing a virtual `DataTree` with `region` or `append_dim` (forwarded to each node) previously always failed with `ContainsGroupError`, because every group was unconditionally created; the existing groups are now opened instead.
+  By [Aaron Spring](https://github.com/aaronspring).
+
 ### Documentation
 
 ### Internal changes

virtualizarr/tests/test_writers/test_icechunk.py

@@ -996,9 +996,6 @@ def test_write_region_unaligned_chunks_raises(
                 write_session.store, region={"y": "auto", "x": "auto"}
             )
 
-    @pytest.mark.xfail(
-        reason="This doesn't work in xarray either, maybe not even intended?",
-    )
     def test_write_datatree_region(
         self,
         icechunk_repo: "Repository",

virtualizarr/writers/icechunk.py

@@ -238,7 +238,12 @@ def get_store_path(subtree, vdt) -> StorePath:
 
     # TODO this serial loop could be slow writing lots of groups to high-latency store, see https://github.com/pydata/xarray/issues/9455
     for store_path, vds in paths_and_virtual_datasets:
-        group = Group.from_store(store=store_path, zarr_format=3)
+        if kwargs.get("append_dim") or kwargs.get("region"):
+            # both require the group (and arrays) to already exist
+            group = Group.open(store=store_path, zarr_format=3)
+        else:
+            # create the group if it doesn't already exist
+            group = Group.from_store(store=store_path, zarr_format=3)
 
         write_virtual_dataset_to_icechunk_group(
             vds=vds,