Close [AI-86](https://linear.app/affine-design/issue/AI-86)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->
## Summary by CodeRabbit
- **New Features**
- Introduced AI model switching in chat, allowing users to select from multiple AI models during conversations.
- Added a floating menu for easy AI model selection within the chat interface.
- Enabled visibility of the AI model switcher through a new experimental feature flag, configurable in workspace settings (canary builds only).
- **Enhancements**
- Improved session management in the chat panel for smoother model switching and state handling.
- Updated localization to support the new AI model switch feature in settings.
- **Bug Fixes**
- None.
- **Chores**
- Registered new components and services to support AI model switching functionality.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->