[BugFix] Preserve diffusion extra_body params for out-of-process CFG Parallel (#4900)

Signed-off-by: princepride <wangzhipeng628@gmail.com>

vllm_omni/entrypoints/async_omni.py

@@ -209,10 +209,13 @@ class AsyncOmni(EngineClient, OmniBase):
 
     def get_diffusion_od_config(self) -> Any | None:
         """Return the diffusion-stage config when the pipeline has one."""
+        saw_diffusion_stage = False
         for stage_client in self.engine.stage_clients:
             if getattr(stage_client, "stage_type", None) != "diffusion":
                 continue
 
+            saw_diffusion_stage = True
+
             od_config = getattr(stage_client, "od_config", None)
             if od_config is not None:
                 return od_config
@@ -222,6 +225,11 @@ class AsyncOmni(EngineClient, OmniBase):
             if od_config is not None:
                 return od_config
 
+        # Out-of-process diffusion clients don't carry od_config (it lives in the
+        # worker); fall back to the engine's model_class_name resolution.
+        if saw_diffusion_stage:
+            return self.engine.get_diffusion_od_config()
+
         return None
 
     @property