修复 AGC 生成准备依赖全量素材库的问题

新增站内与 External v1 单目录读取接口并校验账号归属，保持既有 API 兼容
将已有绑定与 partial 恢复改为按目录 ID 确认，首次建绑不再读取全库
目录缺失或查询失败时停止并保留恢复证据，保持生成幂等与账号隔离
同步 OpenAPI、生成绑定、技术文档及目录读取和生成恢复回归测试

docs/openapi/genarrative-external-v1.openapi.json

@@ -786,6 +786,46 @@
       }
     },
     "/api/external/v1/editor/assets/folders/{folderId}": {
+      "get": {
+        "tags": ["Editor Assets"],
+        "operationId": "getEditorAssetFolder",
+        "summary": "按 ID 读取当前账号素材文件夹",
+        "description": "要求 editor:asset scope。仅按 folderId 读取目录并校验当前账号归属，不读取素材列表或创建默认目录；空目录也可读取。目录不存在返回 404，无权访问返回 403。",
+        "security": [
+          {
+            "ExternalApiKey": []
+          }
+        ],
+        "parameters": [
+          {
+            "$ref": "#/components/parameters/FolderId"
+          }
+        ],
+        "responses": {
+          "200": {
+            "description": "素材文件夹",
+            "content": {
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ExternalEditorAssetFolderResponse"
+                }
+              }
+            }
+          },
+          "400": {
+            "$ref": "#/components/responses/BadRequest"
+          },
+          "401": {
+            "$ref": "#/components/responses/Unauthorized"
+          },
+          "403": {
+            "$ref": "#/components/responses/Forbidden"
+          },
+          "404": {
+            "$ref": "#/components/responses/NotFound"
+          }
+        }
+      },
       "patch": {
         "tags": ["Editor Assets"],
         "operationId": "updateEditorAssetFolder",