[Bugfix] Preserve default diffusion sampling params in default stage (#2780)

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

tests/entrypoints/test_async_omni_diffusion_config.py

@@ -69,6 +69,20 @@ def test_default_stage_config_propagates_ulysses_mode():
     assert parallel_config.ulysses_mode == "advanced_uaa"
 
 
+def test_default_stage_config_includes_default_sampling_params():
+    """Ensure default sampling params survive the default diffusion-stage builder."""
+    stage_cfg = AsyncOmniEngine._create_default_diffusion_stage_cfg(
+        {
+            "default_sampling_params": '{"0": {"generator_device":"cpu", "guidance_scale":7.5}}',
+        }
+    )[0]
+
+    assert stage_cfg["default_sampling_params"] == {
+        "generator_device": "cpu",
+        "guidance_scale": 7.5,
+    }
+
+
 def test_serve_cli_accepts_ulysses_mode():
     """Ensure diffusion serve CLI exposes ulysses_mode and wires it to parallel_config."""
     parser = FlexibleArgumentParser()