增加 xd 网关的断开能力，支持主动解除连接状态

**提交人**: Shinku
**客户端版本**: 0.1.6

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## 使用场景
当前 xd 网关需要支持主动断开，避免网关维持无效连接或在不需要时继续处于连接状态。

## 诉求
- 增加 xd 网关的断开能力
- 断开后网关状态需要及时更新为未连接或已断开
- 断开后的网关不应继续参与后续调用或模型选择
- 为后续重新连接保留明确且可预期的状态流转

## 建议方案
- 在 xd 网关的管理入口增加断开操作
- 断开时清理对应连接态、会话态或相关缓存
- 在路由或选模逻辑中识别断开状态，避免继续使用该网关
- 明确断开后的 UI / 配置反馈，保证用户能感知当前状态
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**OS**: darwin arm64 (25.5.0)
**界面语言**: zh-CN

Start by locating the xd gateway management entry and the route or model-selection logic that uses gateway connection state. Trace how connection, session, and cache state are represented, including the existing reconnect flow. Done means users can disconnect an xd gateway, see its updated UI or configuration status, and the disconnected gateway is excluded from later calls and model selection.

Written by the indexing model from the issue text.