[Bugfix][Full Duplex] Preserve shared runtime compatibility

Signed-off-by: Sy03 <1370724210@qq.com>

tests/entrypoints/openai/test_duplex_protocol.py

@@ -462,6 +462,16 @@ def test_minicpmo_native_capabilities_separate_model_state_from_core_kv_lease():
     assert caps["stage_handoff_transport"] == "scheduler_data_plane"
 
 
+def test_duplex_overlap_policy_auto_remains_a_compatibility_value():
+    from vllm_omni.experimental.fullduplex.openai.protocol import (
+        DuplexOverlapPolicy,
+        DuplexSessionConfig,
+    )
+
+    assert DuplexOverlapPolicy.AUTO.value == "auto"
+    assert DuplexSessionConfig._normalize_overlap_policy("auto") == "auto"
+
+
 def test_minicpmo_native_capabilities_do_not_overclaim_single_session_deployment():
     caps = DuplexCapabilities.minicpmo45_native(max_sessions=1).as_dict()