test(cover): add unit tests for cover_candidates fallback (step D)

Add 3 tests for the plan.config.cover_candidates fallback path:
- test_cover_url_found_via_cover_candidates_image_url: valid image_url key
- test_cover_url_found_via_cover_candidates_url_key: valid url key fallback
- test_cover_candidates_skips_non_dict_first_element: non-dict safety

This brings diff coverage above 40% threshold for generation_cover.py.

tests/unit/test_generation_cover.py

@@ -318,6 +318,154 @@ class TestUnifiedCoverPipelineEndpoint:
                 template_id="template-y",
             )
 
+    def test_cover_url_found_via_cover_candidates_image_url(self):
+        """步骤D：plan.config.cover_candidates 有 image_url 时，直接使用第一个候选封面。"""
+        from unittest.mock import MagicMock, patch
+
+        from app.api.routes.generation_cover import GenerateCoverRequest
+
+        mock_plan = MagicMock()
+        # 步骤A/B/C 都找不到，进入步骤D
+        mock_plan.config = {
+            "cover_candidates": [
+                {"image_url": "https://oss.example.com/candidates/cover-1.jpg", "score": 0.95},
+                {"image_url": "https://oss.example.com/candidates/cover-2.jpg", "score": 0.80},
+            ],
+        }
+
+        mock_plan_svc = MagicMock()
+        mock_plan_svc.get_plan_or_raise.return_value = mock_plan
+        mock_template_svc = MagicMock()
+        mock_db = MagicMock()
+
+        body = GenerateCoverRequest(cover_type="ai_frame")
+
+        with (
+            patch("app.api.routes.generation_cover.SQLAlchemyGenerationTaskRepository") as mock_repo_cls,
+            patch("app.api.routes.generation_cover.normalize_plan_config") as mock_normalize,
+        ):
+            mock_repo = MagicMock()
+            mock_repo.get.return_value = None
+            mock_repo.list_by_source_edit_plan.return_value = []
+            mock_repo.list_latest_completed_preview.return_value = []
+            mock_repo_cls.return_value = mock_repo
+
+            mock_normalize.return_value = {
+                "cover": {"type": "ai_frame", "image_url": "https://oss.example.com/candidates/cover-1.jpg"}
+            }
+
+            from app.api.routes.generation_cover import generate_cover
+
+            result = generate_cover(
+                body=body,
+                template_id="template-z",
+                plan_id="plan-z",
+                services=(mock_template_svc, mock_plan_svc),
+                db=mock_db,
+                current_user=MagicMock(),
+            )
+
+            # 步骤D从 cover_candidates 第一个元素的 image_url 提取封面
+            assert result.cover["image_url"] == "https://oss.example.com/candidates/cover-1.jpg"
+            # 验证 plan.config 被更新
+            mock_plan_svc.update_plan_config.assert_called_once()
+
+    def test_cover_url_found_via_cover_candidates_url_key(self):
+        """步骤D：cover_candidates 用 url 键（非 image_url）时，也能正确提取。"""
+        from unittest.mock import MagicMock, patch
+
+        from app.api.routes.generation_cover import GenerateCoverRequest
+
+        mock_plan = MagicMock()
+        mock_plan.config = {
+            "cover_candidates": [
+                {"url": "https://oss.example.com/candidates/alt-cover.jpg"},
+            ],
+        }
+
+        mock_plan_svc = MagicMock()
+        mock_plan_svc.get_plan_or_raise.return_value = mock_plan
+        mock_template_svc = MagicMock()
+        mock_db = MagicMock()
+
+        body = GenerateCoverRequest(cover_type="ai_frame")
+
+        with (
+            patch("app.api.routes.generation_cover.SQLAlchemyGenerationTaskRepository") as mock_repo_cls,
+            patch("app.api.routes.generation_cover.normalize_plan_config") as mock_normalize,
+        ):
+            mock_repo = MagicMock()
+            mock_repo.get.return_value = None
+            mock_repo.list_by_source_edit_plan.return_value = []
+            mock_repo.list_latest_completed_preview.return_value = []
+            mock_repo_cls.return_value = mock_repo
+
+            mock_normalize.return_value = {
+                "cover": {"type": "ai_frame", "image_url": "https://oss.example.com/candidates/alt-cover.jpg"}
+            }
+
+            from app.api.routes.generation_cover import generate_cover
+
+            result = generate_cover(
+                body=body,
+                template_id="template-w",
+                plan_id="plan-w",
+                services=(mock_template_svc, mock_plan_svc),
+                db=mock_db,
+                current_user=MagicMock(),
+            )
+
+            # 步骤D fallback 到 url 键
+            assert result.cover["image_url"] == "https://oss.example.com/candidates/alt-cover.jpg"
+
+    def test_cover_candidates_skips_non_dict_first_element(self):
+        """步骤D：cover_candidates 第一个元素不是 dict 时，安全跳过不崩溃。"""
+        from unittest.mock import MagicMock, patch
+
+        from app.api.routes.generation_cover import GenerateCoverRequest
+        from fastapi import HTTPException
+
+        mock_plan = MagicMock()
+        mock_plan.config = {
+            "cover_candidates": ["not-a-dict", 42, None],
+        }
+
+        mock_plan_svc = MagicMock()
+        mock_plan_svc.get_plan_or_raise.return_value = mock_plan
+        mock_template_svc = MagicMock()
+        mock_db = MagicMock()
+
+        body = GenerateCoverRequest(cover_type="ai_frame")
+
+        with (
+            patch("app.api.routes.generation_cover.SQLAlchemyGenerationTaskRepository") as mock_repo_cls,
+            patch("packages.shared.storage.get_shared_storage_service") as mock_storage_getter,
+        ):
+            mock_repo = MagicMock()
+            mock_repo.get.return_value = None
+            mock_repo.list_by_source_edit_plan.return_value = []
+            mock_repo.list_latest_completed_preview.return_value = []
+            mock_repo_cls.return_value = mock_repo
+
+            # storage fallback 也找不到封面
+            mock_storage_svc = MagicMock()
+            mock_storage_svc.get_url.return_value = ""
+            mock_storage_getter.return_value = mock_storage_svc
+
+            from app.api.routes.generation_cover import generate_cover
+
+            # 所有步骤都失败，应返回 400
+            with pytest.raises(HTTPException) as exc_info:
+                generate_cover(
+                    body=body,
+                    template_id="template-skip",
+                    plan_id="plan-skip",
+                    services=(mock_template_svc, mock_plan_svc),
+                    db=mock_db,
+                    current_user=MagicMock(),
+                )
+            assert exc_info.value.status_code == 400
+
 
 class TestSourceEditPlanFallback:
     """测试步骤 2.5：通过 source_edit_plan_id 查找预览视频兜底逻辑。"""