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__/__snapshots__/copilot.spec.ts.md

@@ -116,6 +116,38 @@ Generated by [AVA](https://avajs.dev).
       },
     ]
 
+> should generate the final message
+
+    [
+      {
+        content: 'hello world',
+        params: {
+          word: 'world',
+        },
+        role: 'system',
+      },
+      {
+        content: 'hello',
+        params: {},
+        role: 'user',
+      },
+      {
+        content: 'world',
+        params: {},
+        role: 'assistant',
+      },
+      {
+        content: 'aaa',
+        params: {},
+        role: 'user',
+      },
+      {
+        content: 'bbb',
+        params: {},
+        role: 'assistant',
+      },
+    ]
+
 ## should revert message correctly
 
 > should have three messages before revert

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 => {

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

@@ -410,6 +410,27 @@ test('should be able to fork chat session', async t => {
     'should fork new session with same params'
   );
 
+  // fork session without latestMessageId
+  const forkedSessionId3 = await session.fork({
+    userId,
+    sessionId,
+    ...commonParams,
+  });
+
+  // fork session with wrong latestMessageId
+  await t.throwsAsync(
+    session.fork({
+      userId,
+      sessionId,
+      latestMessageId: 'wrong-message-id',
+      ...commonParams,
+    }),
+    {
+      instanceOf: Error,
+    },
+    'should not able to fork new session with wrong latestMessageId'
+  );
+
   const cleanObject = (obj: any[]) =>
     JSON.parse(
       JSON.stringify(obj, (k, v) =>
@@ -436,11 +457,17 @@ test('should be able to fork chat session', async t => {
     t.snapshot(cleanObject(finalMessages), 'should generate the final message');
   }
 
+  // check third times forked session
+  {
+    const s3 = (await session.get(forkedSessionId3))!;
+    const finalMessages = s3.finish(params);
+    t.snapshot(cleanObject(finalMessages), 'should generate the final message');
+  }
+
   // check original session messages
   {
-    const s3 = (await session.get(sessionId))!;
-
-    const finalMessages = s3.finish(params);
+    const s4 = (await session.get(sessionId))!;
+    const finalMessages = s4.finish(params);
     t.snapshot(cleanObject(finalMessages), 'should generate the final message');
   }
 

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(
     `