Close [AI-193](https://linear.app/affine-design/issue/AI-193)
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **New Features**
- Added support for streaming structured AI chat responses as objects,
enabling richer and more interactive chat experiences.
- Chat messages now include a new field displaying structured stream
objects, such as reasoning steps, text deltas, tool calls, and tool
results.
- GraphQL APIs and queries updated to expose these structured streaming
objects in chat histories.
- Introduced a new streaming chat endpoint for object-based responses.
- **Bug Fixes**
- Improved error handling for streaming responses to ensure more robust
and informative error reporting.
- **Refactor**
- Centralized and streamlined session preparation and streaming logic
for AI chat providers.
- Unified streaming setup across multiple AI model providers.
- **Tests**
- Extended test coverage for streaming object responses to ensure
reliability and correctness.
- **Documentation**
- Updated type definitions and schemas to reflect new streaming object
capabilities in both backend and frontend code.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
Co-authored-by: DarkSky <25152247+darkskygit@users.noreply.github.com>
6169cdab3a