Adds no cover on some defensive error handling, cover a few other cases more directly

src/documents/search/_backend.py

@@ -251,7 +251,7 @@ class TantivyBackend:
         Safe to call multiple times - subsequent calls are no-ops.
         """
         if self._index is not None:
-            return
+            return  # pragma: no cover
         if self._path is not None:
             self._index = open_or_rebuild_index(self._path)
         else:
@@ -271,7 +271,7 @@ class TantivyBackend:
     def _ensure_open(self) -> None:
         """Ensure the index is open before operations."""
         if self._index is None:
-            self.open()
+            self.open()  # pragma: no cover
 
     def _build_tantivy_doc(
         self,
@@ -559,7 +559,7 @@ class TantivyBackend:
                         if notes_snippet:
                             highlights["notes"] = str(notes_snippet)
 
-                except Exception:
+                except Exception:  # pragma: no cover
                     logger.debug("Failed to generate highlights for doc %s", doc_id)
 
             hits.append(
@@ -796,7 +796,7 @@ class TantivyBackend:
                 writer.add_document(doc)
             writer.commit()
             new_index.reload()
-        except BaseException:
+        except BaseException:  # pragma: no cover
             # Restore old index on failure so the backend remains usable
             self._index = old_index
             self._schema = old_schema
@@ -832,7 +832,7 @@ def get_backend() -> TantivyBackend:
     with _backend_lock:
         # Double-check after acquiring lock — another thread may have beaten us
         if _backend is not None and _backend_path == current_path:
-            return _backend
+            return _backend  # pragma: no cover
 
         if _backend is not None:
             _backend.close()

src/documents/tests/search/test_backend.py

@@ -280,6 +280,12 @@ class TestMoreLikeThis:
         # private_doc is owned by other, so viewer cannot see it
         assert 53 not in returned_ids
 
+    def test_document_not_in_index_returns_empty(self, backend: TantivyBackend):
+        """more_like_this for a doc_id absent from the index must return empty results."""
+        results = backend.more_like_this(doc_id=9999, user=None, page=1, page_size=10)
+        assert results.hits == []
+        assert results.total == 0
+
 
 class TestSingleton:
     """Test get_backend() and reset_backend() singleton lifecycle."""

src/documents/tests/search/test_query.py

@@ -11,6 +11,9 @@ import pytest
 import tantivy
 import time_machine
 
+from documents.search._query import _date_only_range
+from documents.search._query import _datetime_range
+from documents.search._query import _rewrite_compact_date
 from documents.search._query import build_permission_filter
 from documents.search._query import normalize_query
 from documents.search._query import parse_user_query
@@ -154,6 +157,10 @@ class TestCreatedDateField:
         assert lo == "2025-12-01T00:00:00Z"
         assert hi == "2026-01-01T00:00:00Z"
 
+    def test_unknown_keyword_raises(self) -> None:
+        with pytest.raises(ValueError, match="Unknown keyword"):
+            _date_only_range("bogus_keyword", UTC)
+
 
 class TestDateTimeFields:
     """
@@ -258,6 +265,10 @@ class TestDateTimeFields:
         assert lo == "2025-12-01T00:00:00Z"
         assert hi == "2026-01-01T00:00:00Z"
 
+    def test_unknown_keyword_raises(self) -> None:
+        with pytest.raises(ValueError, match="Unknown keyword"):
+            _datetime_range("bogus_keyword", UTC)
+
 
 class TestWhooshQueryRewriting:
     """All Whoosh query syntax variants must be rewritten to ISO 8601 before Tantivy parses them."""
@@ -363,6 +374,10 @@ class TestWhooshQueryRewriting:
     def test_8digit_invalid_date_passes_through_unchanged(self) -> None:
         assert rewrite_natural_date_keywords("added:20231340", UTC) == "added:20231340"
 
+    def test_compact_14digit_invalid_date_passes_through_unchanged(self) -> None:
+        # Month=13 makes datetime() raise ValueError; the token must be left as-is
+        assert _rewrite_compact_date("20231300120000") == "20231300120000"
+
 
 class TestParseUserQuery:
     """parse_user_query runs the full preprocessing pipeline."""