[frontend] dump openai responses type by alias (#38262)

Signed-off-by: cjackal <44624812+cjackal@users.noreply.github.com>

vllm/entrypoints/openai/responses/protocol.py

@@ -105,8 +105,8 @@ def serialize_message(msg):
     elif hasattr(msg, "to_dict"):
         return msg.to_dict()
     else:
-        # fallback to pyandic dump
-        return msg.model_dump_json()
+        # fallback to pydantic dump
+        return msg.model_dump_json(by_alias=True)
 
 
 def serialize_messages(msgs):