stream_options.include_usage: false still returns usage in streamed chunks

[Hritik003]

Describe the bug
When using the streaming API with stream: true and stream_options.include_usage: false, the streamed responses still include the "usage" field in each data chunk.
This is inconsistent with the documented or expected behavior — setting include_usage: false should suppress the "usage" field in the streamed response chunks.

---

To Reproduce
Use the /v1/completions endpoint with the following payload (or equivalent in go-openai):

{
  "model": "gemma-2",
  "stream": true,
  "stream_options": {
    "include_usage": false
  },
  "prompt": "Write something interesting."
}

Observe that each data: chunk still includes:

"usage": {
  "prompt_tokens": 0,
  "completion_tokens": 0,
  "total_tokens": 0,
  "prompt_tokens_details": null,
  "completion_tokens_details": null
}

Also tested with stream_options.include_usage: true, and both cases show "usage" in stream chunks.

---

Expected behavior
If stream_options.include_usage is set to false, the streamed data chunks should omit the "usage" field entirely or return it as null. Including usage despite explicitly disabling it violates the expected contract of the API.

---

Screenshots/Logs
Example log:

data: {"id":"cmpl-a16ec15821d444e8a22372900e99cc7e","object":"text_completion","created":1750151236,"model":"gemma-2","choices":[{"text":"writing","index":0,"finish_reason":""}],"usage":{"prompt_tokens":0,"completion_tokens":0,"total_tokens":0}}

---

Environment (please complete the following information):

• go-openai version: v1.40.1
• Go version: 1.21
• OpenAI API version: v1
• OS: macOS

---

Additional context
The bug is confusing for consumers trying to reduce response payloads or suppress token accounting per stream chunk. This appears to be a server-side issue or a client-side mishandling of response parsing.

---

[Hritik003]

Important

• Fix for removing usage in every stream chunk response. #1022