[Bugfix] fix: return 400 for unsupported multi-image edits such as Qwen-Image-Layered (#2298)

Signed-off-by: David Chen <530634352@qq.com>

tests/entrypoints/test_async_omni_diffusion.py

@@ -0,0 +1,15 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+import pytest
+
+from vllm_omni.entrypoints.async_omni_diffusion import AsyncOmniDiffusion
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+
+def test_get_diffusion_od_config_returns_direct_config():
+    diffusion = object.__new__(AsyncOmniDiffusion)
+    diffusion.od_config = object()
+
+    assert diffusion.get_diffusion_od_config() is diffusion.od_config