feat(core): support fork session without latestMessageId (#12587)

Close [AI-86](https://linear.app/affine-design/issue/AI-86)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

- **New Features**
  - Improved chat session forking to allow creating a fork without specifying the latest message, enabling more flexible session management.

- **Bug Fixes**
  - Forking a chat session with an invalid latest message ID now correctly returns an error.

- **Tests**
  - Added and updated test cases to cover session forking with missing or invalid latest message IDs, ensuring robust behavior in these scenarios.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/backend/server/src/__tests__/utils/copilot.ts

@@ -57,7 +57,7 @@ export async function forkCopilotSession(
   workspaceId: string,
   docId: string,
   sessionId: string,
-  latestMessageId: string
+  latestMessageId?: string
 ): Promise<string> {
   const res = await app.gql(
     `