fix: guard hed_standard import for compat with released dandischema

hed_standard does not exist in dandischema < 0.12.2, so gate the import
on _SCHEMA_BAREASSET_HAS_DATASTANDARD (all new symbols ship together).
HED detection is skipped when unavailable.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: yarikoptic

dandi/files/bids.py

@@ -12,14 +12,18 @@
     BareAsset,
     StandardsType,
     bids_standard,
-    hed_standard,
     ome_ngff_standard,
 )
 
 import dandi
 from dandi.bids_validator_deno import bids_validate
 
 from .bases import GenericAsset, LocalFileAsset, NWBAsset, _SCHEMA_BAREASSET_HAS_DATASTANDARD
+
+if _SCHEMA_BAREASSET_HAS_DATASTANDARD:
+    from dandischema.models import hed_standard
+else:
+    hed_standard = None  # type: ignore[assignment]
 from .zarr import ZarrAsset
 from ..consts import ZARR_MIME_TYPE, dandiset_metadata_file
 from ..metadata.core import add_common_metadata, prepare_metadata
@@ -234,7 +238,7 @@ def get_metadata(
             _add_standard(metadata, bids_standard)
             return metadata
         _add_standard(metadata, bids_standard, version=desc.get("BIDSVersion"))
-        if hed_version := desc.get("HEDVersion"):
+        if hed_standard and (hed_version := desc.get("HEDVersion")):
             # HEDVersion can be a string or list.
             # List form: ["8.2.0", "sc:1.0.0"] where first element is base
             # HED version and subsequent "prefix:version" entries are library