fix(tests): prefix unused _upserted vars per RUF059

Two test functions in _grid_walk_category heartbeat tests unpack
(upserted, requests) tuple but only assert on requests count.
Ruff RUF059 flags unused upserted — prefix with underscore.

backend/tests/services/test_cadastre_bulk.py

@@ -1162,7 +1162,7 @@ async def test_grid_walk_emits_heartbeat_callbacks() -> None:
 
     progress_states: list[dict[str, Any]] = []
 
-    upserted, requests = await _grid_walk_category(
+    _upserted, requests = await _grid_walk_category(
         db=db,
         client=client,
         quarter="66:41:0303161",
@@ -1197,7 +1197,7 @@ async def test_grid_walk_no_heartbeat_when_callback_none() -> None:
     client.wms_feature_info = AsyncMock(return_value=[])
 
     # Не передаём update_progress — должно не падать
-    upserted, requests = await _grid_walk_category(
+    _upserted, requests = await _grid_walk_category(
         db=db,
         client=client,
         quarter="66:41:0303161",