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__/copilot.e2e.ts

@@ -281,7 +281,7 @@ test('should fork session correctly', async t => {
   const assertForkSession = async (
     workspaceId: string,
     sessionId: string,
-    lastMessageId: string,
+    lastMessageId: string | undefined,
     error: string,
     asserter = async (x: any) => {
       const forkedSessionId = await x;
@@ -330,6 +330,27 @@ test('should fork session correctly', async t => {
     );
   }
 
+  // should be able to fork session without latestMessageId (copy all messages)
+  {
+    forkedSessionId = await assertForkSession(
+      id,
+      sessionId,
+      undefined,
+      'should be able to fork session without latestMessageId'
+    );
+  }
+
+  // should not be able to fork session with wrong latestMessageId
+  {
+    await assertForkSession(id, sessionId, 'wrong-message-id', '', async x => {
+      await t.throwsAsync(
+        x,
+        { instanceOf: Error },
+        'should not able to fork session with wrong latestMessageId'
+      );
+    });
+  }
+
   {
     const u2 = await app.signupV1('u2@affine.pro');
     await assertForkSession(id, sessionId, randomUUID(), '', async x => {