feat(anthropic): add opt-in native passthrough

[srnnkls]

Thanks for the feedback, and sorry my earlier description made the motivation hard to find.

Why

I use the proxy for multiple backends, but sometimes need to switch a task back to native Claude. Today that means changing the client setup or bypassing the proxy, which also loses the proxy's single model-routing path and monitor history.

This adds an opt-in Anthropic provider. With aliasProvider: anthropic, Claude model IDs use the credentials already sent by Claude Code, while GPT, Kimi, Grok, OpenCode Go, and Cursor IDs keep their existing routes.

What changes

• Relays the original Messages request and stream to Anthropic, preserving native fields and thinking blocks.
• Keeps requests visible in the existing monitor and handles token counting.
• Stores no Anthropic credentials; authentication stays with Claude Code.

Nothing changes for existing users unless they select aliasProvider: anthropic.

Verification

Local mock-server tests cover buffered and streaming responses, errors, token counting, credential forwarding, and provider switching. The full Rust test suite and lint checks pass.

[srnnkls]

Is anybody caring about PRs on this project?

[raine]

If you want your PR merged, I'd recommend a different strategy.

You could for example try a concise human readable explanation why you think it would be a good addition to the tool