fix: react_agent.py needs tavily_search_api_key

[MichaelClifford]

What does this PR do?

This PR adds provider_data={"tavily_search_api_key": os.getenv("TAVILY_SEARCH_API_KEY")} to the client definition in examples/agents/react_agent.py. Without it, users can get the following error if they have not previously added the provider data to their server:

llama_stack_client.BadRequestError: Error code: 400 - {'detail': 'Invalid value: Pass Search provider\'s API Key in the header X-LlamaStack-Provider-Data as { "tavily_search_api_key": <your api key>}'}

Feature/Issue validation/testing/test plan

• Test A
  I started a fresh Llama Stack server and ran examples/agents/react_agent.py as is, and I received the error above.

• Test B
  I started a fresh Llama Stack server and ran examples/agents/react_agent.py with the changes in this PR and the example ran as expected.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue? Please add a link
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• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

Thanks for contributing 🎉!