[Refactor 1/N] Centralize Omni config resolution (#5140)

Signed-off-by: Chenguang ZHENG <645327136@qq.com>

tests/diffusion/test_diffusion_step_pipeline.py

@@ -15,6 +15,7 @@ from pytest_mock import MockerFixture
 
 import vllm_omni.diffusion.worker.diffusion_model_runner as model_runner_module
 from tests.helpers.mark import hardware_test
+from vllm_omni.config.config_factory import StageConfigFactory
 from vllm_omni.diffusion.data import DiffusionOutput
 from vllm_omni.diffusion.diffusion_engine import DiffusionEngine
 from vllm_omni.diffusion.diffusion_kv.config import DiffusionKVCacheMode
@@ -46,7 +47,6 @@ from vllm_omni.diffusion.worker.diffusion_model_runner import DiffusionModelRunn
 from vllm_omni.diffusion.worker.diffusion_worker import DiffusionWorker
 from vllm_omni.diffusion.worker.input_batch import InputBatch
 from vllm_omni.diffusion.worker.utils import RunnerOutput, StepRequestState
-from vllm_omni.engine.async_omni_engine import AsyncOmniEngine
 from vllm_omni.errors import OmniClientError
 from vllm_omni.inputs.data import OmniDiffusionSamplingParams
 from vllm_omni.platforms import current_omni_platform
@@ -1182,7 +1182,7 @@ class TestSupportedPipelines:
     """Step-execution protocol checks for supported pipelines."""
 
     def test_default_stage_config_includes_step_execution(self):
-        stage_cfg = AsyncOmniEngine._create_default_diffusion_stage_cfg(
+        stage_cfg = StageConfigFactory.create_default_diffusion(
             {
                 "step_execution": True,
             }