Remove llama-guard in Cerebras template & improve agent test (#798)

# What does this PR do?

- fix cerebras template
- fix agent test case without shields

## Test Plan

<img width="1261" alt="image"
src="https://github.com/user-attachments/assets/04381f85-9192-4fc6-984b-c9bec99bdb82"
/>

```
llama stack run ./llama_stack/templates/cerebras/run.yaml 

LLAMA_STACK_BASE_URL="http://localhost:8321" pytest -v tests/client-sdk/ --html=report.html --self-contained-html
```

## 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/templates/cerebras/run.yaml

@@ -32,12 +32,6 @@ providers:
         type: sqlite
         namespace: null
         db_path: ${env.SQLITE_STORE_DIR:~/.llama/distributions/cerebras}/faiss_store.db
-  - provider_id: chromadb
-    provider_type: remote::chromadb
-    config: {}
-  - provider_id: pgvector
-    provider_type: remote::pgvector
-    config: {}
   agents:
   - provider_id: meta-reference
     provider_type: inline::meta-reference
@@ -111,8 +105,7 @@ models:
   model_id: all-MiniLM-L6-v2
   provider_id: sentence-transformers
   model_type: embedding
-shields:
-- shield_id: meta-llama/Llama-Guard-3-8B
+shields: []
 memory_banks: []
 datasets: []
 scoring_fns: []