[bugfix] fix meta-reference agents w/ safety multiple model loading pytest (#694)

# What does this PR do?

- Fix broken pytest for meta-reference's agents
- Safety model needs to be registered to a different provider id from
inference model in order to be recognized



## Test Plan
```
torchrun $CONDA_PREFIX/bin/pytest -v -s llama_stack/providers/tests/agents/test_agents.py -m "meta_reference" --safety-shield meta-llama/Llama-Guard-3-1B --inference-model meta-llama/Llama-3.1-8B-Instruct
```
**Before**
<img width="845" alt="image"
src="https://github.com/user-attachments/assets/83818fe1-2179-4e9c-a753-bf1472a2f01d"
/>



**After**
<img width="851" alt="image"
src="https://github.com/user-attachments/assets/1cf8124b-14e2-47bf-80fd-ef8b4b3f6fd9"
/>


**Other test not broken**
```
pytest -v -s llama_stack/providers/tests/agents/test_agents.py -m "together" --safety-shield meta-llama/Llama-Guard-3-8B --inference-model meta-llama/Llama-3.1-405B-Instruct-FP8
```

## Sources

Please link relevant resources if necessary.


## 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/agents/fixtures.py

@@ -81,14 +81,28 @@ async def agents_stack(request, inference_model, safety_shield):
     inference_models = (
         inference_model if isinstance(inference_model, list) else [inference_model]
     )
-    models = [
-        ModelInput(
-            model_id=model,
-            model_type=ModelType.llm,
-            provider_id=providers["inference"][0].provider_id,
+
+    # NOTE: meta-reference provider needs 1 provider per model, lookup provider_id from provider config
+    model_to_provider_id = {}
+    for provider in providers["inference"]:
+        if "model" in provider.config:
+            model_to_provider_id[provider.config["model"]] = provider.provider_id
+
+    models = []
+    for model in inference_models:
+        if model in model_to_provider_id:
+            provider_id = model_to_provider_id[model]
+        else:
+            provider_id = providers["inference"][0].provider_id
+
+        models.append(
+            ModelInput(
+                model_id=model,
+                model_type=ModelType.llm,
+                provider_id=provider_id,
+            )
         )
-        for model in inference_models
-    ]
+
     models.append(
         ModelInput(
             model_id="all-MiniLM-L6-v2",