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/tests/eval/fixtures.py

@@ -40,14 +40,30 @@ async def eval_stack(request):
 
     providers = {}
     provider_data = {}
-    for key in ["datasetio", "eval", "scoring", "inference"]:
+    for key in [
+        "datasetio",
+        "eval",
+        "scoring",
+        "inference",
+        "agents",
+        "safety",
+        "memory",
+    ]:
         fixture = request.getfixturevalue(f"{key}_{fixture_dict[key]}")
         providers[key] = fixture.providers
         if fixture.provider_data:
             provider_data.update(fixture.provider_data)
 
     test_stack = await construct_stack_for_test(
-        [Api.eval, Api.datasetio, Api.inference, Api.scoring],
+        [
+            Api.eval,
+            Api.datasetio,
+            Api.inference,
+            Api.scoring,
+            Api.agents,
+            Api.safety,
+            Api.memory,
+        ],
         providers,
         provider_data,
     )