Add support for adaptive thinking and effort parameter (Claude 4.6)

[RezEnayati]

Claude Opus 4.6 and Sonnet 4.6 introduce adaptive thinking (thinking: {type: "adaptive"}) as the recommended thinking mode. The current ThinkingParameters class only supports the now-deprecated type: "enabled" with
budget_tokens.

What's needed

Adaptive thinking type — A Type property on ThinkingParameters to support "adaptive" where Claude dynamically decides when and how much to
think:

{"thinking": {"type": "adaptive"}}

Effort parameter — A new top-level effort parameter (low, medium, high, max) to control thinking depth. Now GA, no beta header required:

{"thinking": {"type": "adaptive"}, "effort": "medium"}

IChatClient extensions — e.g. options.WithAdaptiveThinking() and options.WithAdaptiveThinking(ThinkingEffort.Medium)

References

- https://platform.claude.com/docs/en/about-claude/models/whats-new-claude-4-6
- https://platform.claude.com/docs/en/build-with-claude/adaptive-thinking
- https://platform.claude.com/docs/en/build-with-claude/effort

[tghamm]

#183 shipping today.