chore: correcting openai stream error handling

[dannykopping]

Oversight on my part, I think I stashed some changes and the incomplete impl landed.

In any case, it's working now:

 echo 'what is my coder username?' | OPENAI_BASE_URL="http://localhost:3000/api/experimental/aibridge/openai/v1" llm -m gpt-5 -u --key $KEY
Error: POST "https://api.openai.com/v1/chat/completions": 400 Bad Request {
    "message": "Invalid schema for function 'bmcp_coder_coder_workspace_ls': schema must be a JSON Schema of 'type: \"object\"', got 'type: \"None\"'.",
    "type": "invalid_request_error",
    "param": "tools[0].function.parameters",
    "code": "invalid_function_parameters"
  }