fix: Allow bytes to be nullable on ContainerFileResponse

[knash94]

What:

• Bug Fix
• New Feature

Description:

This PR fixes an issue where OpenAI sometimes responds with the bytes being null instead of a integer in the response when retrieving a container file. I've simply allowed the bytes property to be nullable and added an additional test.

Example request & response showcasing null response

curl https://api.openai.com/v1/containers/cntr_68efad24888881938f8d8a661b5036450ac07bb92e293373/files/cfile_68efad3233308191ae2aea6fdc172940 \
  -H "Authorization: Bearer $OPENAI_API_KEY"

{
  "id": "cfile_68efad3233308191ae2aea6fdc172940",
  "object": "container.file",
  "created_at": 1760537906,
  "bytes": null,
  "container_id": "cntr_68efad24888881938f8d8a661b5036450ac07bb92e293373",
  "path": "/mnt/data/dummy_risk_priority_bar_chart.png",
  "source": "assistant"
}%   

[iBotPeaches]

thanks!