Fix: ensure index dir exists before open, fix test isolation gaps

- `open_or_rebuild_index` now calls `index_dir.mkdir(parents=True, exist_ok=True)`
  so a missing index directory is created on demand rather than crashing on
  `iterdir()` inside `wipe_index`
- `TestTagHierarchy.setUp` calls `super().setUp()` so `DirectoriesMixin` runs
  and `self.dirs` is set before teardown tries to clean up
- `test_search_more_like` d4 content changed to words with no overlap with d2/d3
  to avoid spurious MLT hits from shared stop words at `min_doc_frequency=1`

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

src/documents/search/_schema.py

@@ -161,6 +161,7 @@ def open_or_rebuild_index(index_dir: Path | None = None) -> tantivy.Index:
     """
     if index_dir is None:
         index_dir = settings.INDEX_DIR
+    index_dir.mkdir(parents=True, exist_ok=True)
     if needs_rebuild(index_dir):
         wipe_index(index_dir)
         idx = tantivy.Index(build_schema(), path=str(index_dir))

src/documents/tests/test_api_search.py

@@ -768,8 +768,8 @@ class TestDocumentSearchApi(DirectoriesMixin, APITestCase):
             checksum="C",
         )
         d4 = Document.objects.create(
-            title="Monty Python & the Holy Grail",
-            content="And now for something completely different",
+            title="Quarterly Report",
+            content="quarterly revenue profit margin earnings growth",
             pk=4,
             checksum="ABC",
         )

src/documents/tests/test_tag_hierarchy.py

@@ -11,10 +11,12 @@ from documents.models import WorkflowAction
 from documents.models import WorkflowTrigger
 from documents.serialisers import TagSerializer
 from documents.signals.handlers import run_workflows
+from documents.tests.utils import DirectoriesMixin
 
 
-class TestTagHierarchy(APITestCase):
+class TestTagHierarchy(DirectoriesMixin, APITestCase):
     def setUp(self) -> None:
+        super().setUp()
         self.user = User.objects.create_superuser(username="admin")
         self.client.force_authenticate(user=self.user)