[Bugfix] Keep client error status in the single-GPU diffusion executor (#8474)

Signed-off-by: Wenbo Ji <36562829+fusheng-ji@users.noreply.github.com>

tests/diffusion/test_diffusion_model_runner.py

@@ -1452,3 +1452,27 @@ def test_execute_model_batch_cancellation_preserves_live_peer(monkeypatch, cance
             assert pipeline.last_batch is None
     finally:
         registry.close()
+
+
+@pytest.mark.core_model
+@pytest.mark.cpu
+def test_non_step_fallback_keeps_client_error_status():
+    """Step mode's full-forward fallback must not drop a request error's 4xx status."""
+    from vllm_omni.errors import OmniClientError
+
+    runner = _make_runner(cache_backend=None, cache_backend_name="none")
+    runner.execute_model = Mock(side_effect=OmniClientError("bad request option", status_code=400))
+    request = _make_request()
+    scheduler_output = SimpleNamespace(
+        scheduled_new_reqs=[SimpleNamespace(request_id=request.request_id, req=request, diffusion_kv_metadata=None)],
+        scheduled_cached_reqs=SimpleNamespace(request_ids=[]),
+        kv_prefetch_job=None,
+    )
+
+    result = DiffusionModelRunner._execute_non_step_requests(runner, scheduler_output)
+
+    output = result.get_request_output(request.request_id)
+    assert output.finished is True
+    assert output.result.error == "bad request option"
+    assert output.result.error_status_code == 400
+    assert output.result.error_type == "BadRequestError"