refactor: update config access from dict to attribute-style in test case

Author: jackzhxng

extension/llm/export/test/test_export_llm.py

@@ -142,13 +142,13 @@ def test_with_config_and_cli(self, mock_export_llama: MagicMock) -> None:
             mock_export_llama.assert_called_once()
             called_config = mock_export_llama.call_args[0][0]
             self.assertEqual(
-                called_config["base"]["model_class"], "stories110m"
+                called_config.base.model_class, "stories110m"
             )  # Override from CLI.
             self.assertEqual(
-                called_config["model"]["dtype_override"].value, "fp16"
+                called_config.model.dtype_override.value, "fp16"
             )  # From yaml.
             self.assertEqual(
-                called_config["backend"]["xnnpack"]["enabled"],
+                called_config.backend.xnnpack.enabled,
                 True,  # Override from CLI.
             )
         finally: