[Bugfix] Restore BAGEL CFG KV transfers (#5620)

Signed-off-by: lishunyang12 <lishunyang03@gmail.com>
Co-authored-by: lishunyang12 <lishunyang03@gmail.com>

tests/core/sched/test_omni_ar_scheduler_kv_transfer.py

@@ -0,0 +1,57 @@
+from types import SimpleNamespace
+
+import pytest
+from vllm import SamplingParams
+from vllm.v1.engine import EngineCoreRequest
+
+from vllm_omni.core.sched.omni_ar_scheduler import OmniARScheduler
+from vllm_omni.engine.async_engine_utils import apply_omni_final_stage_metadata
+from vllm_omni.engine.serialization import deserialize_additional_information
+
+pytestmark = [pytest.mark.core_model, pytest.mark.cpu]
+
+
+def _make_engine_request() -> EngineCoreRequest:
+    return EngineCoreRequest(
+        request_id="req",
+        prompt_token_ids=[1],
+        mm_features=None,
+        sampling_params=SamplingParams(max_tokens=1),
+        pooling_params=None,
+        arrival_time=0.0,
+        lora_request=None,
+        cache_salt=None,
+        data_parallel_rank=None,
+    )
+
+
+def _request_omits_kv_transfer(*, force_kv_transfer: bool) -> tuple[bool, dict]:
+    tagged = apply_omni_final_stage_metadata(
+        _make_engine_request(),
+        final_stage_id=0,
+        force_kv_transfer=force_kv_transfer,
+    )
+    scheduler = OmniARScheduler.__new__(OmniARScheduler)
+    scheduler._omits_kv_transfer_cache = {}
+    request = SimpleNamespace(
+        request_id="req",
+        additional_information=tagged.additional_information,
+    )
+    result = scheduler._request_omits_kv_transfer_to_next_stage(request)
+    metadata = deserialize_additional_information(tagged.additional_information)
+    return result, metadata
+
+
+def test_stage_zero_request_omits_kv_transfer():
+    omits_transfer, metadata = _request_omits_kv_transfer(force_kv_transfer=False)
+
+    assert omits_transfer
+    assert "omni_force_kv_transfer" not in metadata
+
+
+def test_cfg_companion_forces_kv_transfer_without_downstream_payload():
+    omits_transfer, metadata = _request_omits_kv_transfer(force_kv_transfer=True)
+
+    assert not omits_transfer
+    assert metadata["omni_final_stage_id"] == 0
+    assert metadata["omni_force_kv_transfer"] is True