Support Anthropic interleaved thinking with tool use

[markpollack]

support Claude 4 models feature of interleaved thinking with tool use, allowing the model to reason between tool calls.

See https://docs.anthropic.com/en/docs/build-with-claude/extended-thinking#extended-thinking-with-tool-use

To support this feature will require

• Store thinking blocks, their signatures, and the order in which they appeared
• Maintain this state between API calls in a multi-turn conversation
• Add support for the interleaved-thinking-2025-05-14 beta header
• Enforce compatible tool_choice settings (auto or none only)
• Allow budget_tokens to exceed max_tokens for interleaved thinking