fix(kook): KOOK 机器人鉴权用 Authorization: Bot <token> 而非 Bearer

httpclient 硬编码 Bearer，导致 KOOK bot token 被当 OAuth2 用户令牌发，
/ gateway/index 返回 401『登录环境异常』。按 KOOK reference 鉴权规范，
给共享 httpclient 增加可配置 authScheme（默认 Bearer），KOOK 用 Bot。

- httpclient: 新增 WithAuthScheme 选项 + authScheme 字段，do() 按 scheme 组装
- kook/adapter: 装配时传 WithAuthScheme("Bot")
- 同步修正 config.go 注释与 send/identity 测试断言

plugins/channels/kook/config.go

@@ -3,7 +3,7 @@
 // modes: a WebSocket-gateway mode (events over a gateway WebSocket with
 // hello/heartbeat/event/resume signaling) and a Webhook mode (KOOK POSTs event
 // callbacks to a Cyrene gateway route). Outbound messages are always sent over a
-// bearer-authenticated REST API.
+// bot-token-authenticated REST API (`Authorization: Bot <token>`).
 //
 // The adapter conforms to internal/channel/adapter.PlatformAdapter, translating
 // native KOOK events into the normalized adapter.UnifiedMessage model and