Implement comprehensive support for Claude thinking models with interleaved
thinking in multi-turn conversations:
- Add signature caching system to preserve and restore thinking block
signatures across conversation turns, preventing "invalid signature" errors
- Enable real-time SSE streaming with immediate forwarding of thinking tokens
- Add interleaved-thinking-2025-05-14 beta header for Claude thinking models
- Implement smart system hints to encourage thinking during tool use
- Add VALIDATED mode for tool calling on Claude models
- Ensure output token limits accommodate thinking budgets
- Filter and sanitize thinking blocks, removing SDK-injected cache_control
- Add comprehensive test suites for auth, cache, and request-helpers modules
- Update build config to exclude test files from production builds
- Document streaming and thinking features in README
