fix(design): connect video preparation and playable history

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

@@ -203,6 +203,23 @@ describe('AI design V2 Main route boundary', () => {
         clientOperationId: 'operation-1',
       },
     },
+    {
+      body: {
+        kind: 'apply_input',
+        sessionId: 'session-1',
+        expectedDirectionRevision: 4,
+        clientOperationId: 'operation-1',
+        input: { kind: 'prepare_generation' },
+      },
+      expected: {
+        kind: 'apply_input',
+        workspaceId: 'workspace-1',
+        sessionId: 'session-1',
+        expectedDirectionRevision: 4,
+        clientOperationId: 'operation-1',
+        input: { kind: 'prepare_generation', operations: [] },
+      },
+    },
     {
       body: {
         kind: 'confirm_generation',