feat(core): add stream object api (#12841)

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>

packages/backend/server/src/plugins/copilot/session.ts

@@ -282,6 +282,7 @@ export class ChatSessionService {
           await tx.aiSessionMessage.createMany({
             data: state.messages.map(m => ({
               ...m,
+              streamObjects: m.streamObjects || undefined,
               attachments: m.attachments || undefined,
               params: omit(m.params, ['docs']) || undefined,
               sessionId,
@@ -512,6 +513,7 @@ export class ChatSessionService {
               id: true,
               role: true,
               content: true,
+              streamObjects: true,
               attachments: true,
               params: true,
               createdAt: true,