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_acceptance.py

@@ -245,3 +245,19 @@ class TestMultitokenInNestedOr:
         backend.add_or_update(doc_b)
         matched = _matched_ids(backend, 'tag:"multi word tag" OR title:B')
         assert matched == {doc_a.pk, doc_b.pk}
+
+
+class TestUnregisteredIdFieldFoldsToLiteralText:
+    """tag_id, owner_id, etc. are intentionally excluded from the
+    FieldRegistry - whoosh-compat parity leniency folds them into a literal
+    text search rather than raising a diagnostic/400 (see docs/usage.md's
+    advanced-search section). Prove the fold is inert against real data, not
+    just that parsing doesn't raise."""
+
+    def test_tag_id_query_matches_nothing(
+        self,
+        backend: TantivyBackend,
+        indexed_documents: dict[str, int],
+    ) -> None:
+        matched = _matched_ids(backend, "tag_id:5")
+        assert matched == set()

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)