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/providers/provider.ts

@@ -33,6 +33,7 @@ import {
   ModelInputType,
   type PromptMessage,
   PromptMessageSchema,
+  StreamObject,
 } from './types';
 
 @Injectable()
@@ -225,6 +226,17 @@ export abstract class CopilotProvider<C = any> {
     options?: CopilotChatOptions
   ): AsyncIterable<string>;
 
+  streamObject(
+    _model: ModelConditions,
+    _messages: PromptMessage[],
+    _options?: CopilotChatOptions
+  ): AsyncIterable<StreamObject> {
+    throw new CopilotProviderNotSupported({
+      provider: this.type,
+      kind: 'object',
+    });
+  }
+
   structure(
     _cond: ModelConditions,
     _messages: PromptMessage[],