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/schema.prisma

@@ -414,14 +414,15 @@ model AiPrompt {
 }
 
 model AiSessionMessage {
-  id          String       @id @default(uuid()) @db.VarChar
-  sessionId   String       @map("session_id") @db.VarChar
-  role        AiPromptRole
-  content     String       @db.Text
-  attachments Json?        @db.Json
-  params      Json?        @db.Json
-  createdAt   DateTime     @default(now()) @map("created_at") @db.Timestamptz(3)
-  updatedAt   DateTime     @updatedAt @map("updated_at") @db.Timestamptz(3)
+  id            String       @id @default(uuid()) @db.VarChar
+  sessionId     String       @map("session_id") @db.VarChar
+  role          AiPromptRole
+  content       String       @db.Text
+  streamObjects Json?        @db.Json
+  attachments   Json?        @db.Json
+  params        Json?        @db.Json
+  createdAt     DateTime     @default(now()) @map("created_at") @db.Timestamptz(3)
+  updatedAt     DateTime     @updatedAt @map("updated_at") @db.Timestamptz(3)
 
   session AiSession @relation(fields: [sessionId], references: [id], onDelete: Cascade)