Python: fix dict access on ToolInvocation dataclass in test_tools.py

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: SteveSandersonMS

python/e2e/test_tools.py

@@ -110,7 +110,7 @@ def db_query(params: DbQueryParams, invocation: ToolInvocation) -> list[City]:
             assert params.query.table == "cities"
             assert params.query.ids == [12, 19]
             assert params.query.sortAscending is True
-            assert invocation["session_id"] == expected_session_id
+            assert invocation.session_id == expected_session_id
 
             return [
                 City(countryId=19, cityName="Passos", population=135460),