omit server name when created with config

src/tracking/mlflow.py

@@ -48,13 +48,14 @@ class MlflowTracker:
                 "Install with: qc-cli[mlflow]"
             ) from e
 
-        if not cfg.mlflow.tracking_server_name:
-            raise RuntimeError("mlflow.tracking_server_name is required when MLflow is enabled.")
+        tracking_server_name = cfg.effective_mlflow_tracking_server_name
+        if not tracking_server_name:
+            raise RuntimeError("MLflow tracking server name could not be resolved.")
 
         tracking_uri = aws_mlflow.get_tracking_server_arn(
             cfg.aws.region,
             cfg.aws.profile,
-            cfg.mlflow.tracking_server_name,
+            tracking_server_name,
         )
         mlflow.set_tracking_uri(tracking_uri)
         mlflow.set_experiment(cfg.mlflow.experiment_name)