[Config Refactor] Derive deploy override fields from stage config (#3162)

Signed-off-by: xiaohajiayou <923390377@qq.com>

tests/entrypoints/test_async_omni_diffusion_config.py

@@ -4,6 +4,7 @@
 import pytest
 from vllm.utils.argparse_utils import FlexibleArgumentParser
 
+from vllm_omni.config.stage_config import deploy_override_field_names
 from vllm_omni.engine.async_omni_engine import AsyncOmniEngine
 from vllm_omni.entrypoints.cli.serve import OmniServeCommand, _create_default_diffusion_stage_cfg
 
@@ -30,6 +31,15 @@ def test_default_stage_config_includes_cache_backend():
     assert engine_args["model_stage"] == "diffusion"
 
 
+def test_default_stage_config_ignores_none_deploy_overrides():
+    """Ensure nullified deploy override defaults do not alter diffusion defaults."""
+    baseline = AsyncOmniEngine._create_default_diffusion_stage_cfg({})[0]
+    nullified_overrides = {name: None for name in deploy_override_field_names()}
+    stage_cfg = AsyncOmniEngine._create_default_diffusion_stage_cfg(nullified_overrides)[0]
+
+    assert stage_cfg == baseline
+
+
 def test_default_cache_config_used_when_missing():
     """Ensure default cache_config is synthesized when only backend is given."""
     stage_cfg = AsyncOmniEngine._create_default_diffusion_stage_cfg(