minor fix up react_agent

[yanxi0830]

What does this PR do?

• address comments in [RFC] Client Agent SDK OutputParser #121

Test Plan

• see feat: ReACT agent example llama-stack-apps#166

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,
  Pull Request section?
• Updated relevant documentation.
• Wrote necessary unit or integration tests.