Fix invalid message format when using OpenAIServerModel

[sijunhe]

This PR fixes #583

The bug is the fact that due to OpenAI SDK uses flat message format without images but nested message format with image. The current code always use nested message, which results in this bug.

without image

client.chat.completions.create(
    messages=[
        {
            "role": "user",
            "content": "Say this is a test",
        }
    ],
    model="gpt-4o",
)

with image

client.chat.completions.create(
    model="gpt-4o-mini",
    messages=[
        {
            "role": "user",
            "content": [
                {"type": "text", "text": prompt},
                {
                    "type": "image_url",
                    "image_url": {"url": f"data:{img_type};base64,{img_b64_str}"},
                },
            ],
        }
    ],
)