Changes
PR #1698 by ksylvan: Enable One Million Token Context Beta Feature for Sonnet-4
- Chore: upgrade anthropic-sdk-go to v1.9.1 and add beta feature support for context-1m
- Add modelBetas map for beta feature configuration
- Implement context-1m-2025-08-07 beta for Claude Sonnet 4
- Add beta header support with fallback handling
- Preserve existing beta headers in OAuth transport