[Config Refactor] sentinel default precedence (#3078)

Signed-off-by: lishunyang <lishunyang12@163.com>
Signed-off-by: xiaohajiayou <923390377@qq.com>
Co-authored-by: lishunyang <lishunyang12@163.com>

tests/engine/test_single_stage_mode.py

@@ -422,6 +422,39 @@ class TestSingleStageModeDetection:
         )
         assert engine._single_stage_id_filter is None
 
+    def test_engine_args_create_only_forwards_explicit_fields(self, mocker: MockerFixture):
+        from vllm_omni.engine.arg_utils import OmniEngineArgs
+
+        captured: dict[str, Any] = {}
+
+        def fake_resolve(self, model: str, kwargs: dict[str, Any]):
+            captured.update(kwargs)
+            return "/fake/path", [_make_stage_cfg(0)]
+
+        mocker.patch.object(AsyncOmniEngine, "_resolve_stage_configs", fake_resolve)
+        mocker.patch.object(AsyncOmniEngine, "_bootstrap_orchestrator")
+        mock_thread_cls = mocker.patch("threading.Thread")
+        mock_future_cls = mocker.patch("concurrent.futures.Future")
+        mock_future = mocker.Mock()
+        mock_future.result.return_value = mocker.Mock()
+        mock_future_cls.return_value = mock_future
+        mock_thread = mocker.Mock()
+        mock_thread.is_alive.return_value = False
+        mock_thread_cls.return_value = mock_thread
+
+        ea = OmniEngineArgs.create(model="ignored", gpu_memory_utilization=0.5)
+        AsyncOmniEngine(model="fake-model", engine_args=ea)
+
+        assert captured["gpu_memory_utilization"] == 0.5
+        assert "model" not in captured
+        assert "max_num_seqs" not in captured
+
+    def test_bare_engine_args_rejected(self, mocker: MockerFixture):
+        from vllm_omni.engine.arg_utils import OmniEngineArgs
+
+        with pytest.raises(TypeError, match="OmniEngineArgs.create"):
+            self._make_engine_no_thread(mocker, engine_args=OmniEngineArgs(model="fake-model"))
+
     def test_master_address_and_port_stored(self, mocker: MockerFixture):
         engine = self._make_engine_no_thread(
             mocker,