fix playground for v1 (#799)

# What does this PR do?

- update playground callsites for v1 api changes

## Test Plan

```
cd llama_stack/distribution/ui
streamlit run app.py
```


https://github.com/user-attachments/assets/eace11c6-600a-42dc-b4e7-6948a706509f




## 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/distribution/ui/page/playground/chat.py

@@ -121,7 +121,7 @@ if prompt := st.chat_input("Example: What is Llama Stack?"):
         if stream:
             for chunk in response:
                 if chunk.event.event_type == "progress":
-                    full_response += chunk.event.delta
+                    full_response += chunk.event.delta.text
                 message_placeholder.markdown(full_response + "▌")
             message_placeholder.markdown(full_response)
         else: