feat(simple_metrics): thread model_type from model_config to activity

Extracts model_type from model_config (optional, defaults to None when
absent) and passes it to calculate_simple_metrics. Required for the r2
lock - is_r2_supported() needs model_type to decide whether to compute r2.

Updates workflow tests to expect the new key in the activity-call dict.

tests/laborious/workflows/test_simple_metrics.py

@@ -103,6 +103,7 @@ async def test_run(workflow_mock: AsyncMock, simple_metrics: SimpleMetrics):
             'target_data': target_data,
             'metrics': input_data['metrics'],
             'interval_minutes': input_data['interval_minutes'],
+            'model_type': None,
         },
         retry_policy=ANY,
         start_to_close_timeout=ANY,
@@ -209,6 +210,7 @@ async def test_run_default_metrics(workflow_mock: AsyncMock, simple_metrics: Sim
             'target_data': target_data,
             'metrics': ['rmse', 'mse', 'mae', 'r2'],  # Default value
             'interval_minutes': input_data['interval_minutes'],
+            'model_type': None,
         },
         retry_policy=ANY,
         start_to_close_timeout=ANY,