fix: stabilize ai design history switching

tests/unit/image-workspace-api.test.ts

@@ -104,6 +104,11 @@ describe('AI design renderer API boundary', () => {
       'client-conversation-one',
     );
     await fetchImageWorkspaceConversation('workspace/one', 'conversation/one');
+    await fetchImageWorkspaceConversation(
+      'workspace/one',
+      'conversation/one',
+      'older page/+',
+    );
 
     expect(hostApiFetchMock).toHaveBeenNthCalledWith(
       1,
@@ -121,6 +126,11 @@ describe('AI design renderer API boundary', () => {
       '/api/works/image-workspace/workspaces/workspace%2Fone/conversations/conversation%2Fone',
       {},
     );
+    expect(hostApiFetchMock).toHaveBeenNthCalledWith(
+      3,
+      '/api/works/image-workspace/workspaces/workspace%2Fone/conversations/conversation%2Fone?before=older%20page%2F%2B',
+      {},
+    );
   });
 
   it('sends conversation turns with the current turn revision and no provider settings', async () => {