fixes tests & move braintrust api_keys to request headers

[yanxi0830]

What does this PR do?

• braintrust scoring provider requires OPENAI_API_KEY env variable to be set
• move this to be able to be set as request headers (e.g. like together / fireworks api keys)
• fixes pytest with agents dependency

Test Plan

E2E

llama stack run 

scoring:
  - provider_id: braintrust-0
    provider_type: inline::braintrust
    config: {}

Client

self.client = LlamaStackClient(
    base_url=os.environ.get("LLAMA_STACK_ENDPOINT", "http://localhost:5000"),
    provider_data={
        "openai_api_key": os.environ.get("OPENAI_API_KEY", ""),
    },
)

• run llama-stack-client eval run_scoring

Unit Test

pytest -v -s -m meta_reference_eval_together_inference eval/test_eval.py

pytest -v -s -m braintrust_scoring_together_inference scoring/test_scoring.py --env OPENAI_API_KEY=$OPENAI_API_KEY

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Ran pre-commit to handle lint / formatting issues.
• Read the contributor guideline,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.