Fix Qwen3-Omni MoE backend selection

Signed-off-by: hsliuustc0106 <liuhongsheng4@huawei.com>

tests/engine/test_stage_engine_args.py

@@ -104,7 +104,6 @@ _OMNI_ONLY_LLM_STAGE_ENGINE_FIELDS = frozenset(
 def _stable_engine_arg_environment(monkeypatch, tmp_path):
     from vllm_omni import platforms
 
-    monkeypatch.delenv("VLLM_USE_FLASHINFER_MOE_FP16", raising=False)
     xcodec_model = tmp_path / "xcodec-model"
     xcodec_model.mkdir()
     monkeypatch.setenv("XCODEC1_PATH", str(xcodec_model))
@@ -137,6 +136,25 @@ def _stable_engine_arg_environment(monkeypatch, tmp_path):
         monkeypatch.setitem(sys.modules, worker_module.__name__, worker_module)
 
 
+def test_qwen3_omni_defaults_to_triton_moe_backend():
+    engine_args = {"model_arch": "Qwen3OmniMoeForConditionalGeneration"}
+
+    stage_init_utils._maybe_set_qwen3_omni_moe_backend(engine_args)
+
+    assert engine_args["moe_backend"] == "triton"
+
+
+def test_qwen3_omni_preserves_explicit_moe_backend():
+    engine_args = {
+        "model_arch": "Qwen3OmniMoeForConditionalGeneration",
+        "moe_backend": "flashinfer",
+    }
+
+    stage_init_utils._maybe_set_qwen3_omni_moe_backend(engine_args)
+
+    assert engine_args["moe_backend"] == "flashinfer"
+
+
 def _engine_arg_inputs(tmp_path: Path) -> tuple[PipelineConfig, DeployConfig, str]:
     parent_model = tmp_path / "parent-model"
     stage_model = tmp_path / "stage-model"