feat: remove special handling of builtin::rag tool

[ehhuang]

Summary:

Lets the model decide which tool it needs to call to respond to a query.

Test Plan:

LLAMA_STACK_CONFIG=fireworks pytest -s -v tests/client-sdk/ --safety-shield meta-llama/Llama-Guard-3-8B

Also evaluated on a small benchmark with 20 questions from HotpotQA. With this PR and some prompting, the performance is 77% recall compared to 50% currently.

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• feat: allow specifying specific tool within toolgroup #1239
• -> feat: remove special handling of builtin::rag tool #1015

[hardikjshah]

Also reminder - we will need to update getting_started and other places where we have RAG agent.

[hardikjshah]

can we update the tests to not hard code but manage other models properly please ?