Migrate Anthropic Proxy to CloudConnector::Tokens

What does this MR do and why?

Migrate Anthropic Proxy to the updated Token interface.

This is a refactoring and should not result in any user-facing changes.
The change is behind the anthropic_proxy_new_tokens_path feature flag.

References

Issue: #561289 (closed)

How to set up and validate locally

❓ I am not sure how to validate it locally.

My initial instinct was to play with Generate Issue Description feature, but:

• I am not sure if it always goes through Anthropic Proxy
• I can't trigger it in UX locally despite that: 1. I run in SaaS; 2. I have Duo Enterprise; 3. I enabled beta/experimental features for the given group; 4. Even on master.

I will try with other UPs but I will also double-check with AI experts.

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Related to #561289 (closed)