fix: wrong response header for non streamed responses

[louis-jan]

Describe Your Changes

{
  "messages": [
    {
      "content": "Hello!",
      "role": "user"
    }
  ],
  "model": "tinyllama-1.1b",
  "stream": false,
  "max_tokens": 2048
}

Fixes Issues

• bug: response header wrong for non streamed responses (no chunks) #2560
• bug: when use Jan with mistral server REST API and open ai client Jan does not return JSON #2429

Self Checklist

• Added relevant comments, esp in complex areas
• Updated docs (for bug fixes / features)
• Created issues for follow-up changes or refactoring needed