Implement claude citation api for RAG

Anthropic released a new Citation API for claude. You can find a discussion of it here: https://simonwillison.net/2025/Jan/24/anthropics-new-citations-api/

And the official docs here: https://docs.anthropic.com/en/docs/build-with-claude/citations

Adding this to claudette would allow for improved RAG with citation grounding and open many interesting usecases for users. It is also officially supported by anthropic and would allow for us to say we continue to keep up with the official anthropic api

Start with Anthropic's official Citations API documentation and the linked discussion, then trace claudette's existing Claude request and response handling. Done means users can use citation grounding through the library while preserving existing API behavior and covering the supported citation response.

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