[ResponseAPI] Simplify input/output message serialization (#26620)

Signed-off-by: Jialin Ouyang <Jialin.Ouyang@gmail.com>

tests/entrypoints/openai/test_response_api_with_harmony.py

@@ -12,6 +12,8 @@ from openai_harmony import (
     Message,
 )
 
+from vllm.entrypoints.openai.protocol import serialize_message, serialize_messages
+
 from ...utils import RemoteOpenAIServer
 
 MODEL_NAME = "openai/gpt-oss-20b"
@@ -758,3 +760,32 @@ async def test_output_messages_enabled(client: OpenAI, model_name: str, server):
     assert response.status == "completed"
     assert len(response.input_messages) > 0
     assert len(response.output_messages) > 0
+
+
+def test_serialize_message() -> None:
+    dict_value = {"a": 1, "b": "2"}
+    assert serialize_message(dict_value) == dict_value
+
+    msg_value = {
+        "role": "assistant",
+        "name": None,
+        "content": [{"type": "text", "text": "Test 1"}],
+        "channel": "analysis",
+    }
+    msg = Message.from_dict(msg_value)
+    assert serialize_message(msg) == msg_value
+
+
+def test_serialize_messages() -> None:
+    assert serialize_messages(None) is None
+    assert serialize_messages([]) is None
+
+    dict_value = {"a": 3, "b": "4"}
+    msg_value = {
+        "role": "assistant",
+        "name": None,
+        "content": [{"type": "text", "text": "Test 2"}],
+        "channel": "analysis",
+    }
+    msg = Message.from_dict(msg_value)
+    assert serialize_messages([msg, dict_value]) == [msg_value, dict_value]