refactor: consolidate generation workflow helpers

backend/tests/test_stories.py

@@ -428,6 +428,33 @@ class TestUnifiedGenerations:
         assert data["generation_status"] == "narrative_ready"
         assert data["image_status"] == "not_requested"
 
+    def test_create_story_generation_image_failure(
+        self,
+        auth_client: TestClient,
+        mock_text_provider,
+    ):
+        with patch("app.services.story_service.generate_image", new_callable=AsyncMock) as mock_img:
+            mock_img.side_effect = Exception("Image API error")
+
+            response = auth_client.post(
+                "/api/generations",
+                json={
+                    "output_mode": "story",
+                    "type": "keywords",
+                    "data": "小兔子, 森林",
+                    "generate_images": True,
+                },
+            )
+
+        assert response.status_code == 200
+        data = response.json()
+        assert data["image_url"] is None
+        assert data["generation_status"] == "degraded_completed"
+        assert data["image_status"] == "failed"
+        assert data["audio_status"] == "not_requested"
+        assert "Image API error" in data["errors"]["image"]
+        assert "Image API error" in data["last_error"]
+
     def test_create_storybook_generation_success(self, auth_client: TestClient):
         with patch(
             "app.services.story_service.generate_storybook",