fix(valkey): skip async mixin tests gracefully on older haystack versions

Use pytest.importorskip so the test file is skipped instead of failing
with ImportError when running with haystack < 2.27.0 (lowest-direct-deps step).

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

Author: SyedShahmeerAli12

integrations/valkey/tests/test_document_store_async.py

@@ -9,7 +9,10 @@
 import pytest
 import pytest_asyncio
 from haystack.dataclasses import ByteStream, Document
-from haystack.testing.document_store import (
+
+pytest.importorskip("haystack.testing.document_store_async", reason="Requires haystack with async mixin test classes")
+
+from haystack.testing.document_store import (  # noqa: E402
     CountDocumentsByFilterAsyncTest,
     CountUniqueMetadataByFilterAsyncTest,
     UpdateByFilterAsyncTest,