test(mcp): patch _validate_update_against_dataset on the module, not the re-exported function

`test_dataset_id_passed_to_update_command` patched the symbol via the string
path "superset.mcp_service.chart.tool.update_chart._validate_update_against_dataset".
Because tool/__init__.py re-exports the update_chart *function*, mock's importer
resolves ...tool.update_chart to the function and raises AttributeError, failing
the test deterministically on master. Use patch.object(update_chart_module, ...)
to match the pattern the rest of the file already uses.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/unit_tests/mcp_service/chart/tool/test_update_chart.py

@@ -1480,9 +1480,8 @@ class TestUpdateChartDatasetIdIntegration:
         "superset.commands.chart.update.UpdateChartCommand",
         new_callable=Mock,
     )
-    @patch(
-        "superset.mcp_service.chart.tool.update_chart._validate_update_against_dataset",
-        return_value=None,
+    @patch.object(
+        update_chart_module, "_validate_update_against_dataset", return_value=None
     )
     @patch("superset.daos.chart.ChartDAO.find_by_id", new_callable=Mock)
     @patch("superset.db.session")