refactor: minor cleanup from final whoosh-compat migration review

- Update stale test comments in test_query.py that described string
  rewriting / raw-query fallback behavior that no longer exists post
  whoosh-compat migration; rename
  test_date_rewriting_applied_before_tantivy_parse to
  test_date_keyword_resolves_without_raising to match.
- views.py: move the local MultipleSearchQueryErrors import up into the
  existing local-import block near the top of list(), consistent with
  the other documents.search imports there, instead of importing it
  again inside the except SearchQueryError clause.
- test_api_search.py: assert response.status_code explicitly before
  indexing into response.data["results"] in
  test_search_added_previous_month_excludes_next_period_start, and tie
  the xfail marker to AssertionError instead of the incidental KeyError
  that indexing a 400 response's missing "results" key produced.

src/documents/tests/test_api_search.py

@@ -731,7 +731,7 @@ class TestDocumentSearchApi(DirectoriesMixin, APITestCase):
             "which already quotes. CONFIRMED REGRESSION vs whoosh-compat "
             "migration; see task-10-report.md."
         ),
-        raises=KeyError,
+        raises=AssertionError,
     )
     def test_search_added_previous_month_excludes_next_period_start(self) -> None:
         """
@@ -769,6 +769,10 @@ class TestDocumentSearchApi(DirectoriesMixin, APITestCase):
             tick=False,
         ):
             response = self.client.get("/api/documents/?query=added:previous month")
+        assert response.status_code == 200, (
+            f"expected a successful search response, got {response.status_code}: "
+            f"{response.data!r}"
+        )
         results = response.data["results"]
 
         self.assertEqual(len(results), 1)