fix(rag-example): add provider_id to avoid llama_stack_client 400 error

[fulvius31]

What does this PR do?

Add provider_id to avoid errors using the rag example with llama_stack_client

llama_stack_client.BadRequestError: Error code: 400 - {'detail': 'Invalid value: No provider specified and multiple providers available. Please specify a provider_id.'}

Test Plan

[Describe the tests you ran to verify your changes with result summaries. Provide clear instructions so the plan can be easily re-executed.]

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[yanxi0830]

could you update the PR title to pass semantic PR title-check?