技能展示适配:支持多结果集(result sets)渲染

- 后端 extensions_config 新增 SkillDisplayResultSetConfig,SkillDisplayConfig 增加 resultSets 字段
- threads 引用响应增加 resultSetKey / resultSetLabel,runtime/references 适配多结果集
- 前端 skill-display 适配器/运行时/类型支持多结果集渲染,聊天框、消息组、技能展示面板相应更新

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

offline-backend-20260512/backend/app/gateway/routers/threads.py

@@ -228,6 +228,8 @@ class ReferenceSourceResponse(BaseModel):
     id: str | None = None
     skillName: str = "backend-reference"
     mode: str = "citation"
+    resultSetKey: str | None = None
+    resultSetLabel: str | None = None
     title: str
     type: str = "result"
     snippet: str = ""