Support [AI-59](https://linear.app/affine-design/issue/AI-59)
<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit
- **New Features**
- Updated the default AI model for chat prompts to use Claude Sonnet 4.
- **Bug Fixes**
- Improved model selection logic to better support reasoning features across more AI models.
- **Tests**
- Enhanced test cases with consistent instructions for response length.
- Skipped certain chat-related tests to refine test suite stability.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->