fixes tests & move braintrust api_keys to request headers (#535)

# 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 
```
```yaml
scoring:
  - provider_id: braintrust-0
    provider_type: inline::braintrust
    config: {}
```

**Client**
```python
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
```
<img width="745" alt="image"
src="https://github.com/user-attachments/assets/68f5cdda-f6c8-496d-8b4f-1b3dabeca9c2">

## 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](https://github.com/meta-llama/llama-stack/blob/main/CONTRIBUTING.md),
      Pull Request section?
- [ ] Updated relevant documentation.
- [ ] Wrote necessary unit or integration tests.

llama_stack/providers/registry/scoring.py

@@ -44,5 +44,6 @@ def available_providers() -> List[ProviderSpec]:
                 Api.datasetio,
                 Api.datasets,
             ],
+            provider_data_validator="llama_stack.providers.inline.scoring.braintrust.BraintrustProviderDataValidator",
         ),
     ]