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/utils.ts

@@ -14,7 +14,7 @@ import {
   createExaCrawlTool,
   createExaSearchTool,
 } from '../tools';
-import { PromptMessage } from './types';
+import { PromptMessage, StreamObject } from './types';
 
 type ChatMessage = CoreUserMessage | CoreAssistantMessage;
 
@@ -387,6 +387,22 @@ export interface CustomAITools extends ToolSet {
 
 type ChunkType = TextStreamPart<CustomAITools>['type'];
 
+export function parseUnknownError(error: unknown) {
+  if (typeof error === 'string') {
+    throw new Error(error);
+  } else if (error instanceof Error) {
+    throw error;
+  } else if (
+    typeof error === 'object' &&
+    error !== null &&
+    'message' in error
+  ) {
+    throw new Error(String(error.message));
+  } else {
+    throw new Error(JSON.stringify(error));
+  }
+}
+
 export class TextStreamParser {
   private readonly CALLOUT_PREFIX = '\n[!]\n';
 
@@ -446,8 +462,8 @@ export class TextStreamParser {
         break;
       }
       case 'error': {
-        const error = chunk.error as { type: string; message: string };
-        throw new Error(error.message);
+        parseUnknownError(chunk.error);
+        break;
       }
     }
     this.lastType = chunk.type;
@@ -490,3 +506,54 @@ export class TextStreamParser {
     return links;
   }
 }
+
+export class StreamObjectParser {
+  public parse(chunk: TextStreamPart<CustomAITools>) {
+    switch (chunk.type) {
+      case 'reasoning':
+      case 'text-delta':
+      case 'tool-call':
+      case 'tool-result': {
+        return chunk;
+      }
+      case 'error': {
+        parseUnknownError(chunk.error);
+        return null;
+      }
+      default: {
+        return null;
+      }
+    }
+  }
+
+  public mergeTextDelta(chunks: StreamObject[]): StreamObject[] {
+    return chunks.reduce((acc, curr) => {
+      const prev = acc.at(-1);
+      switch (curr.type) {
+        case 'reasoning':
+        case 'text-delta': {
+          if (prev && prev.type === curr.type) {
+            prev.textDelta += curr.textDelta;
+          } else {
+            acc.push(curr);
+          }
+          break;
+        }
+        default: {
+          acc.push(curr);
+          break;
+        }
+      }
+      return acc;
+    }, [] as StreamObject[]);
+  }
+
+  public mergeContent(chunks: StreamObject[]): string {
+    return chunks.reduce((acc, curr) => {
+      if (curr.type === 'text-delta') {
+        acc += curr.textDelta;
+      }
+      return acc;
+    }, '');
+  }
+}