Add support for https://github.com/EricLBuehler/mistral.rs

Would be great to add support for https://github.com/EricLBuehler/mistral.rs. This allows the inference to be embedded in the binary.

Look at the existing LLM provider integrations in the codebase, likely under a directory like `langchain_rust/llms/` or `langchain_rust/integrations/`. Understand how the `mistral.rs` crate works and how to embed its inference. The goal is to create a new struct implementing the existing LLM trait, handling the API differences. Check for existing tests for other providers to understand the expected interface.

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