fix: Use re-entrancy and concurrency safe context managers for provider data (#1498)

Concurrent requests should not trample (or reuse) each others' provider
data. Provider data should be scoped to each request.

## Test Plan

Set the uvicorn server to have a single worker process + thread by
updating the config:
```python
    uvicorn_config = {
        ...
        "workers": 1,
        "loop": "asyncio",
    }
```

Then perform the following steps on `origin/main` (without this change).

(1) Run the server using `llama stack run dev` without having
`FIREWORKS_API_KEY` in the environment.

(2) Run a test by specifying the FIREWORKS_API_KEY env var so it gets
stored in the thread local
```
pytest -s -v tests/integration/inference/test_text_inference.py \
    --stack-config http://localhost:8321 \
    --text-model accounts/fireworks/models/llama-v3p1-8b-instruct \
    -k test_text_chat_completion_with_tool_calling_and_streaming \
     --env FIREWORKS_API_KEY=<...>
``` 
Ensure you don't have any other API keys in the environment (otherwise
the bug will not reproduce due to other specifics in our testing code.)
Verify this works.

(3) Run the same command again without specifying FIREWORKS_API_KEY. See
that the request actually succeeds when it *should have failed*.


----
Now do the same tests on this branch, verify step (3) results in
failure.

Finally, run the full `test_text_inference.py` test suite with this
change, verify it succeeds.

llama_stack/providers/remote/inference/fireworks/fireworks.py

@@ -70,8 +70,9 @@ class FireworksInferenceAdapter(ModelRegistryHelper, Inference, NeedsRequestProv
         pass
 
     def _get_api_key(self) -> str:
-        if self.config.api_key is not None:
-            return self.config.api_key.get_secret_value()
+        config_api_key = self.config.api_key.get_secret_value() if self.config.api_key else None
+        if config_api_key:
+            return config_api_key
         else:
             provider_data = self.get_request_provider_data()
             if provider_data is None or not provider_data.fireworks_api_key: