[responsesAPI][bugfix] serialize harmony messages

[qandrew]

Purpose

harmony messages are not being serialized properly, this was the state prior to this PR.

context.parser.messages[0]
Message(author=Author(role=<Role.ASSISTANT: 'assistant'>, name=None), content=[TextContent(text='We need to respond as ChatGPT. The user says "Hello." We respond politely. Possibly ask how can help.')], channel='analysis', recipient=None, content_type=None)
context.parser.messages[0].model_dump_json()
'{"author":{"role":"assistant","name":null},"content":[{}],"channel":"analysis","recipient":null,"content_type":null}'

We fix it by adding a custom serialization method in protocol.py. I filed a issue: openai/harmony#78

Test Plan

added unit tests, and ran locally successfully

server

 CUDA_VISIBLE_DEVICES=2,3 with-proxy vllm serve "/data/users/axia/checkpoints/gpt-oss-120b" -tp 2 --port 20001

client

curl http://localhost:20001/v1/responses   -H "Content-Type: application/json"   -N   -d '{
    "model": "/data/users/axia/checkpoints/gpt-oss-120b",
    "input": [
        {
            "role": "user",
            "content": "Hello."
        }
    ],
    "temperature": 0.7,
    "max_output_tokens": 256,
    "stream": true,
    "enable_response_messages": true
}'


...


event: response.completed
data: {"response":{"id":"resp_f6111e47923e423e9a21c35775af604c","created_at":1759515694,"incomplete_details":null,"instructions":null,"metadata":null,"model":"/data/users/axia/checkpoints/gpt-oss-120b","object":"response","output":[{"id":"rs_6f2b920ea35f4bbb99e6ca2cee9580de","summary":[],"type":"reasoning","content":[{"text":"We need to respond as ChatGPT, friendly greeting. Probably ask how can help.","type":"reasoning_text"}],"encrypted_content":null,"status":null},{"id":"msg_6b9222d5e23a46d4becc526cf101d085","content":[{"annotations":[],"text":"Hello! How can I assist you today?","type":"output_text","logprobs":null}],"role":"assistant","status":"completed","type":"message"}],"parallel_tool_calls":true,"temperature":0.7,"tool_choice":"auto","tools":[],"top_p":1.0,"background":false,"max_output_tokens":256,"max_tool_calls":null,"previous_response_id":null,"prompt":null,"reasoning":null,"service_tier":"auto","status":"completed","text":null,"top_logprobs":null,"truncation":"disabled","usage":{"input_tokens":67,"input_tokens_details":{"cached_tokens":0},"output_tokens":36,"output_tokens_details":{"reasoning_tokens":18,"tool_output_tokens":0},"total_tokens":103},"user":null,"input_messages":[{"author":{"role":"system","name":null},"content":[{"model_identity":"You are ChatGPT, a large language model trained by OpenAI.","reasoning_effort":"Medium","conversation_start_date":"2025-10-03","knowledge_cutoff":"2024-06","channel_config":{"valid_channels":["analysis","final"],"channel_required":true},"tools":null}],"channel":null,"recipient":null,"content_type":null},{"author":{"role":"user","name":null},"content":[{"text":"Hello."}],"channel":null,"recipient":null,"content_type":null}],"output_messages":[{"author":{"role":"assistant","name":null},"content":[{"text":"We need to respond as ChatGPT, friendly greeting. Probably ask how can help."}],"channel":"analysis","recipient":null,"content_type":null},{"author":{"role":"assistant","name":null},"content":[{"text":"Hello! How can I assist you today?"}],"channel":"final","recipient":null,"content_type":null}]},"sequence_number":38,"type":"response.completed"}

^ note output_messages has content as text and not null (that was the previous bug).

---

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[qandrew]

cc @lacora , @houseroad , @yeqcharlotte , @alecsolder this is ready for review

[yeqcharlotte]

thanks for adding the tests

[Jialin]

Seems we could consolidate majority of the code (e.g. message to serialized_message mapping) to a separate function? It could allow us to

1. replace list.append with a simple list comprehensive (with better readability and better performance)
2. better individual unit tests.

[Jialin]

Trying to refactor it a bit in #26620