feat(anthropic): new "reasoning level" format

Reference: https://platform.claude.com/docs/en/build-with-claude/adaptive-thinking

The issue references adding a new 'reasoning level' format for Anthropic's Claude. Start by examining the existing Anthropic client integration in the codebase, likely under a module like `rig::providers::anthropic`. Look for where request formats are defined and how other features (like temperature, max_tokens) are structured. The external documentation link provides the spec; implement the new field, ensuring it serializes correctly. Run any existing tests for the Anthropic provider to verify.

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