Fix: fall back to in-memory index when INDEX_DIR does not exist

When open_or_rebuild_index is called and the index directory does not exist,
return a fresh in-memory Tantivy index instead of creating the directory as
a side effect. This prevents workspace contamination during test runs where
INDEX_DIR has not been redirected to a temp directory.

In production the data directory is always created during setup, so disk-
based indexes continue to work normally.

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

src/documents/search/_schema.py

@@ -161,7 +161,8 @@ 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 not index_dir.exists():
+        return tantivy.Index(build_schema())
     if needs_rebuild(index_dir):
         wipe_index(index_dir)
         idx = tantivy.Index(build_schema(), path=str(index_dir))