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/plugins/copilot/resolver.ts

@@ -91,8 +91,9 @@ class ForkChatSessionInput {
   @Field(() => String, {
     description:
       'Identify a message in the array and keep it with all previous messages into a forked session.',
+    nullable: true,
   })
-  latestMessageId!: string;
+  latestMessageId?: string;
 }
 
 @InputType()