fix(kook): 丢弃自我回声消息，防 bot 把自己的回复当入站用户消息

KOOK 网关会把 bot 自己发的消息又推回给 bot(author_id=bot uid)，导致 bot 把自己的
回复当成入站用户消息处理(循环/浪费token)。toUnifiedMessage 计算 senderID 后调用
isSelf(senderID) 比对 a.botID(user/me 解析出的 bot 账号 id)，命中即返回 ok=false 丢弃。
- identity.go: 新增 isSelf(并发安全,botMu)
- message.go: self-echo 丢弃 + debug 日志
- message_test.go: TestToUnified_DropsSelfEcho
验收: go build/vet/test ./... 全绿

plugins/channels/kook/identity.go

@@ -81,6 +81,19 @@ func (a *Adapter) botIdentitySnapshot() (id, username, nickname string) {
 	return a.botID, a.botUsername, a.botNickname
 }
 
+// isSelf reports whether senderID is the bot's own account id. KOOK delivers the
+// bot's own messages back over the gateway, so an inbound event whose author_id equals
+// the bot's user id is a self-echo and must be dropped — otherwise the bridge would
+// process the bot's replies as inbound user messages and loop (or burn tokens).
+func (a *Adapter) isSelf(senderID string) bool {
+	if senderID == "" {
+		return false
+	}
+	a.botMu.RLock()
+	defer a.botMu.RUnlock()
+	return a.botID != "" && senderID == a.botID
+}
+
 // BotIdentity returns the resolved bot account (id/username/nickname). The values
 // are non-sensitive (the bot's public handle and display name) so the admin panel
 // may show them.