test(p0): add unit tests for default template fallback (diff cov gate)

- 覆盖 resolve_draft_plan_id 空tid兜底/400/404/草稿复用/全局模板分支
- 满足 diff coverage ≥40% 门槛

tests/unit/test_default_template_fallback_p0.py

@@ -0,0 +1,146 @@
+"""P0 #1922 默认模板自动兜底单元测试。
+
+覆盖 resolve_draft_plan_id 的核心路径（使用可mock的外部依赖）：
+- 空 tid 时调用 get_or_create_default_template_id 兜底
+- auto_create_default=False 时空tid抛400
+- 模板不存在/无权访问抛404
+- 已有草稿直接返回 plan_id
+- 旧模板（templates表）走 generate_from_template 创建草稿
+
+_get_or_create_default_template_id 是薄SQL封装，其逻辑分支通过依赖注入路径间接覆盖；
+直接SQL分支通过integration test/容器验证，不做单元级mock（避免内部import脆弱mock）。
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+from app.api.routes.templates_editor.dependencies import resolve_draft_plan_id
+
+
+class TestResolveDraftPlanId:
+    """resolve_draft_plan_id 行为。"""
+
+    def _make_services(self, global_tpl=None, draft=None):
+        tpl_svc = MagicMock()
+        plan_svc = MagicMock()
+        tpl_svc.get_template.return_value = global_tpl
+        tpl_svc.get_template_draft.return_value = draft
+        return (tpl_svc, plan_svc)
+
+    def _make_user(self, uid: str = "user-1"):
+        u = MagicMock()
+        u.user.id = uid
+        return u
+
+    def test_empty_tid_auto_fallback_creates_default_then_draft(self):
+        """template_id 为空 + auto_create_default=True：先兜底拿到tid，再走旧模板草稿创建。"""
+        db = MagicMock()
+        services = self._make_services()
+        user = self._make_user()
+        old_tpl = MagicMock()
+        old_tpl.id = "tpl-auto"
+        old_tpl.name = "默认配音模板"
+        old_tpl.mode = "voice_over"
+        old_tpl.segments = [MagicMock(id="seg-1", segment_order=0, duration_min=1.0, duration_max=30.0)]
+        mock_repo = MagicMock()
+        mock_repo.get_active.return_value = old_tpl
+        mock_plan = MagicMock()
+        mock_plan.id = "plan-new"
+        mock_generator = MagicMock()
+        mock_generator.generate_from_template.return_value = {"plan": mock_plan}
+
+        with (
+            patch("app.api.routes.templates_editor.dependencies.SQLAlchemyTemplateRepository", return_value=mock_repo),
+            patch(
+                "app.api.routes.templates_editor.dependencies.get_or_create_default_template_id",
+                return_value="tpl-auto",
+            ) as mock_fb,
+            patch("app.services.plan_generator_service.PlanGeneratorService", return_value=mock_generator),
+        ):
+            plan_id = resolve_draft_plan_id(
+                template_id="",
+                services=services,
+                current_user=user,
+                db=db,
+                auto_create_default=True,
+            )
+
+        assert plan_id == "plan-new"
+        mock_fb.assert_called_once_with(db, "user-1")
+
+    def test_empty_tid_without_auto_raises_400(self):
+        """auto_create_default=False 且 tid 为空 → 400。"""
+        from fastapi import HTTPException
+
+        db = MagicMock()
+        services = self._make_services()
+        with pytest.raises(HTTPException) as exc:
+            resolve_draft_plan_id(
+                template_id="",
+                services=services,
+                current_user=self._make_user(),
+                db=db,
+                auto_create_default=False,
+            )
+        assert exc.value.status_code == 400
+
+    def test_missing_template_raises_404(self):
+        """tid 存在但模板找不到/无权访问 → 404。"""
+        from fastapi import HTTPException
+
+        db = MagicMock()
+        services = self._make_services()  # get_template returns None
+        mock_repo = MagicMock()
+        mock_repo.get_active.return_value = None
+        with patch("app.api.routes.templates_editor.dependencies.SQLAlchemyTemplateRepository", return_value=mock_repo):
+            with pytest.raises(HTTPException) as exc:
+                resolve_draft_plan_id(
+                    template_id="tpl-ghost",
+                    services=services,
+                    current_user=self._make_user(),
+                    db=db,
+                    auto_create_default=False,
+                )
+        assert exc.value.status_code == 404
+
+    def test_existing_draft_returned_directly(self):
+        """已有模板草稿直接返回 plan_id，不重复创建。"""
+        db = MagicMock()
+        draft = MagicMock()
+        draft.id = "plan-existing"
+        services = self._make_services(draft=draft)
+        old_tpl = MagicMock()
+        old_tpl.id = "tpl-1"
+        old_tpl.segments = []
+        mock_repo = MagicMock()
+        mock_repo.get_active.return_value = old_tpl
+        with patch("app.api.routes.templates_editor.dependencies.SQLAlchemyTemplateRepository", return_value=mock_repo):
+            plan_id = resolve_draft_plan_id(
+                template_id="tpl-1",
+                services=services,
+                current_user=self._make_user(),
+                db=db,
+                auto_create_default=False,
+            )
+        assert plan_id == "plan-existing"
+        # 没触发generate
+        services[1].update_plan_config.assert_not_called()
+
+    def test_global_template_creates_draft_via_new_service(self):
+        """全局模板（新系统edit_templates表存在）走tpl_svc.create_template_draft。"""
+        db = MagicMock()
+        services = self._make_services(global_tpl=MagicMock(), draft=None)
+        new_draft = MagicMock()
+        new_draft.id = "plan-global"
+        services[0].create_template_draft.return_value = new_draft
+        plan_id = resolve_draft_plan_id(
+            template_id="tpl-global",
+            services=services,
+            current_user=self._make_user(),
+            db=db,
+            auto_create_default=False,
+        )
+        assert plan_id == "plan-global"
+        services[0].create_template_draft.assert_called_once_with("tpl-global", user_id="user-1")