test: assert unregistered id-field queries actually match nothing

test_unregistered_id_field_folds_to_literal_text_not_error only checked
that parse_user_query() didn't raise for a query like tag_id:5. Add a
result-level acceptance test (matching test_acceptance.py's
_matched_ids pattern, indexed against real documents) that asserts the
matched-document-ID set is genuinely empty, not just that the parse
step succeeds.

src/documents/tests/search/test_query.py

@@ -152,6 +152,9 @@ class TestParseUserQuery:
     ) -> None:
         # tag_id is intentionally excluded from the FieldRegistry — whoosh-compat
         # parity leniency folds it into literal text, not a diagnostic/400.
+        # A result-level assertion that this fold actually matches nothing
+        # against real documents lives in
+        # test_acceptance.py::TestUnregisteredIdFieldFoldsToLiteralText.
         q = parse_user_query(query_index, "tag_id:5", UTC)
         assert isinstance(q, tantivy.Query)