test: 补充plan_id兜底逻辑执行测试，diff覆盖率100%

新增3个端到端测试覆盖generation_tasks.py的兜底关联逻辑：
- test_fallback_sets_plan_id_when_empty: 验证空plan_id时自动查找并关联
- test_no_fallback_when_plan_id_already_set: 验证已有plan_id时不触发兜底
- test_fallback_handles_exception_gracefully: 验证查找异常不阻塞主流程

总计10个回归测试，diff覆盖率从7%提升到100%（14/14行全覆盖）

tests/unit/test_three_bugs_fix.py

@@ -187,3 +187,229 @@ class TestPlanIdFallback:
         repo = SQLAlchemyEditPlanRepository(mock_db)
         result = repo.list_by_template("tpl-1", limit=20)
         assert result == []
+
+
+class TestPlanIdFallbackExecution:
+    """Test that the fallback logic actually executes when source_edit_plan_id is empty."""
+
+    @staticmethod
+    def _make_mock_task(source_edit_plan_id=""):
+        t = MagicMock()
+        t.id = "task-1"
+        t.project_id = "proj-1"
+        t.asset_library_id = ""
+        t.strategy_id = "one_take"
+        t.voice_library_id = ""
+        t.template_id = "tpl-1"
+        t.asset_ids = []
+        t.title_ids = []
+        t.voice_ids = []
+        t.source_edit_plan_id = source_edit_plan_id
+        t.asset_select_mode = "manual"
+        t.batch_id = ""
+        t.video_title = ""
+        t.resolution = ""
+        t.bgm_config = None
+        t.is_preview = False
+        t.source_task_id = ""
+        t.output_width = 1280
+        t.output_height = 720
+        t.cover_url = ""
+        t.custom_title = ""
+        t.title_config = {}
+        t.logs = "[]"
+        t.status = "pending"
+        t.progress = 0.0
+        t.error_message = ""
+        t.error_info = None
+        t.created_at = "2026-01-01T00:00:00Z"
+        t.updated_at = "2026-01-01T00:00:00Z"
+        t.started_at = None
+        t.completed_at = None
+        t.created_by_user_id = "user-1"
+        t.auto_retry_enabled = False
+        t.auto_retry_max = 0
+        t.auto_retry_count = 0
+        return t
+
+    @staticmethod
+    def _make_request(source_edit_plan_id="", template_id="tpl-1"):
+        req = MagicMock()
+        req.template_id = template_id
+        req.source_edit_plan_id = source_edit_plan_id
+        req.asset_ids = []
+        req.asset_select_mode = "manual"
+        req.asset_select_count = 0
+        req.voice_library_id = ""
+        req.title_ids = []
+        req.voice_ids = []
+        req.strategy_id = "one_take"
+        req.count = 1
+        req.video_title = ""
+        req.resolution = ""
+        req.bgm_config = None
+        req.auto_retry_enabled = False
+        req.auto_retry_max = 0
+        req.is_preview = False
+        req.source_task_id = ""
+        req.output_width = 0
+        req.output_height = 0
+        req.cover_url = ""
+        req.custom_title = ""
+        req.title_config = {}
+        req.project_id = None
+        req.asset_library_id = None
+        return req
+
+    @patch("apps.api.app.api.routes.generation_tasks._to_generation_task_response")
+    @patch("apps.api.app.api.routes.generation_tasks.safe_enqueue_generation_task")
+    @patch("apps.api.app.api.routes.generation_tasks.CreateGenerationTaskUseCase")
+    @patch("apps.api.app.api.routes.generation_tasks._resolve_project_and_library")
+    def test_fallback_sets_plan_id_when_empty(self, mock_resolve, mock_uc_cls, mock_enqueue, mock_resp_fn):
+        """When source_edit_plan_id is empty, fallback finds plan and sets it."""
+        from apps.api.app.api.routes.generation_tasks import create_generation_task
+
+        mock_resolve.return_value = ("proj-1", None)
+        mock_task = self._make_mock_task(source_edit_plan_id="")
+        mock_uc_cls.return_value.execute.return_value = mock_task
+        mock_enqueue.return_value = True
+        from app.schemas.generation_task import GenerationTaskResponse
+        mock_resp_fn.return_value = GenerationTaskResponse(
+            id="task-1", project_id="proj-1", asset_library_id="",
+            strategy_id="one_take", voice_library_id="", template_id="tpl-1",
+            asset_ids=[], title_ids=[], voice_ids=[], source_edit_plan_id="",
+            asset_select_mode="", batch_id="", video_title="", resolution="",
+            bgm_config={}, is_preview=False, source_task_id="",
+            output_width=1280, output_height=720, cover_url="", custom_title="",
+            title_config={}, logs="[]", status="pending", progress=0.0,
+            error_message="", created_at="2026-01-01T00:00:00Z",
+            updated_at="2026-01-01T00:00:00Z", started_at=None, completed_at=None,
+            created_by_user_id="user-1", auto_retry_enabled=False,
+            auto_retry_max=0, auto_retry_count=0, result_count=0,
+        )
+
+        mock_plan = MagicMock()
+        mock_plan.id = "plan-found-123"
+        mock_plan.created_by_user_id = "user-1"
+
+        mock_plan_repo_cls = MagicMock()
+        mock_plan_repo_instance = MagicMock()
+        mock_plan_repo_instance.list_by_template.return_value = [mock_plan]
+        mock_plan_repo_cls.return_value = mock_plan_repo_instance
+
+        mock_gen_repo = MagicMock()
+        mock_gen_repo.count_pending_by_user.return_value = 0
+        mock_gen_repo.count_pending_total.return_value = 0
+
+        with patch(
+            "packages.adapters.sqlalchemy_impl.edit_plan_repository.SQLAlchemyEditPlanRepository",
+            mock_plan_repo_cls,
+        ):
+            create_generation_task(
+                request=self._make_request(source_edit_plan_id="", template_id="tpl-1"),
+                authenticated_user=MagicMock(user=MagicMock(id="user-1")),
+                generation_task_repository=mock_gen_repo,
+                project_repository=MagicMock(),
+                asset_library_repository=MagicMock(),
+                asset_repository=MagicMock(),
+                db=MagicMock(),
+            )
+
+        assert mock_task.source_edit_plan_id == "plan-found-123"
+        mock_gen_repo.update.assert_called_once_with(mock_task)
+
+    @patch("apps.api.app.api.routes.generation_tasks._to_generation_task_response")
+    @patch("apps.api.app.api.routes.generation_tasks.safe_enqueue_generation_task")
+    @patch("apps.api.app.api.routes.generation_tasks.CreateGenerationTaskUseCase")
+    @patch("apps.api.app.api.routes.generation_tasks._resolve_project_and_library")
+    def test_no_fallback_when_plan_id_already_set(self, mock_resolve, mock_uc_cls, mock_enqueue, mock_resp_fn):
+        """When source_edit_plan_id is already set, fallback should NOT run."""
+        from apps.api.app.api.routes.generation_tasks import create_generation_task
+
+        mock_resolve.return_value = ("proj-1", None)
+        mock_task = self._make_mock_task(source_edit_plan_id="plan-already-set")
+        mock_uc_cls.return_value.execute.return_value = mock_task
+        mock_enqueue.return_value = True
+        from app.schemas.generation_task import GenerationTaskResponse
+        mock_resp_fn.return_value = GenerationTaskResponse(
+            id="task-1", project_id="proj-1", asset_library_id="",
+            strategy_id="one_take", voice_library_id="", template_id="tpl-1",
+            asset_ids=[], title_ids=[], voice_ids=[], source_edit_plan_id="",
+            asset_select_mode="", batch_id="", video_title="", resolution="",
+            bgm_config={}, is_preview=False, source_task_id="",
+            output_width=1280, output_height=720, cover_url="", custom_title="",
+            title_config={}, logs="[]", status="pending", progress=0.0,
+            error_message="", created_at="2026-01-01T00:00:00Z",
+            updated_at="2026-01-01T00:00:00Z", started_at=None, completed_at=None,
+            created_by_user_id="user-1", auto_retry_enabled=False,
+            auto_retry_max=0, auto_retry_count=0, result_count=0,
+        )
+
+        mock_gen_repo = MagicMock()
+        mock_gen_repo.count_pending_by_user.return_value = 0
+        mock_gen_repo.count_pending_total.return_value = 0
+
+        create_generation_task(
+            request=self._make_request(source_edit_plan_id="plan-already-set", template_id="tpl-1"),
+            authenticated_user=MagicMock(user=MagicMock(id="user-1")),
+            generation_task_repository=mock_gen_repo,
+            project_repository=MagicMock(),
+            asset_library_repository=MagicMock(),
+            asset_repository=MagicMock(),
+            db=MagicMock(),
+        )
+
+        assert mock_task.source_edit_plan_id == "plan-already-set"
+        mock_gen_repo.update.assert_not_called()
+
+    @patch("apps.api.app.api.routes.generation_tasks._to_generation_task_response")
+    @patch("apps.api.app.api.routes.generation_tasks.safe_enqueue_generation_task")
+    @patch("apps.api.app.api.routes.generation_tasks.CreateGenerationTaskUseCase")
+    @patch("apps.api.app.api.routes.generation_tasks._resolve_project_and_library")
+    def test_fallback_handles_exception_gracefully(self, mock_resolve, mock_uc_cls, mock_enqueue, mock_resp_fn):
+        """When plan lookup fails, the fallback should not break the main flow."""
+        from apps.api.app.api.routes.generation_tasks import create_generation_task
+
+        mock_resolve.return_value = ("proj-1", None)
+        mock_task = self._make_mock_task(source_edit_plan_id="")
+        mock_uc_cls.return_value.execute.return_value = mock_task
+        mock_enqueue.return_value = True
+        from app.schemas.generation_task import GenerationTaskResponse
+        mock_resp_fn.return_value = GenerationTaskResponse(
+            id="task-1", project_id="proj-1", asset_library_id="",
+            strategy_id="one_take", voice_library_id="", template_id="tpl-1",
+            asset_ids=[], title_ids=[], voice_ids=[], source_edit_plan_id="",
+            asset_select_mode="", batch_id="", video_title="", resolution="",
+            bgm_config={}, is_preview=False, source_task_id="",
+            output_width=1280, output_height=720, cover_url="", custom_title="",
+            title_config={}, logs="[]", status="pending", progress=0.0,
+            error_message="", created_at="2026-01-01T00:00:00Z",
+            updated_at="2026-01-01T00:00:00Z", started_at=None, completed_at=None,
+            created_by_user_id="user-1", auto_retry_enabled=False,
+            auto_retry_max=0, auto_retry_count=0, result_count=0,
+        )
+
+        mock_plan_repo_cls = MagicMock()
+        mock_plan_repo_cls.side_effect = Exception("DB connection error")
+
+        mock_gen_repo = MagicMock()
+        mock_gen_repo.count_pending_by_user.return_value = 0
+        mock_gen_repo.count_pending_total.return_value = 0
+
+        with patch(
+            "packages.adapters.sqlalchemy_impl.edit_plan_repository.SQLAlchemyEditPlanRepository",
+            mock_plan_repo_cls,
+        ):
+            create_generation_task(
+                request=self._make_request(source_edit_plan_id="", template_id="tpl-1"),
+                authenticated_user=MagicMock(user=MagicMock(id="user-1")),
+                generation_task_repository=mock_gen_repo,
+                project_repository=MagicMock(),
+                asset_library_repository=MagicMock(),
+                asset_repository=MagicMock(),
+                db=MagicMock(),
+            )
+
+        # Task should still be created (fallback error doesn't break main flow)
+        assert mock_task.source_edit_plan_id == ""
+        mock_gen_repo.update.assert_not_called()