OpenAI backend validation fails with 401 Unauthorized when API key authentication is required

[mo0ogie]

Describe the bug
The OpenAI backend's validate() method fails when connecting to OpenAI-compatible API endpoints that require authentication on their health check endpoints. The validation request to the /health endpoint does not include the Authorization header, causing a 401 Unauthorized error even when a valid API key is provided.

Expected behavior
When using --backend-type openai_http with a valid API key, the backend validation should successfully authenticate against the remote server's health endpoint.

Actual behavior
httpx.HTTPStatusError: Client error '401 Unauthorized' for url 'https://my-server/health'
RuntimeError: Backend validation request failed. Could not connect to the server or validate the backend

Environment
Include all relevant environment information:
guidellm version: 0.4.0
Python 3.12.12

To Reproduce
Exact steps to reproduce the behavior:

1. Set environment variables for the key and base url (GUIDELLM__OPENAI__API_KEY, BASE_URL)
2. Run the benchmark

guidellm benchmark run \
  --target "$BASE_URL" \ 
  --backend-type openai_http \
  --model "model" \
  --rate-type constant --rate 1 \
  --max-requests 10 \
  --data 'prompt_tokens=256,output_tokens=128'

Additional Context
Refer to #491

[arno4000]

I have currently exactly the same issue.