feat: heal dead sync channels and block deleting system default (Z12h)

Ensure default channel and rebind Agent/Binding on list, ticket, agents/me, and ensure-binding so SyncPage does not stay on deleted-channel errors.

Co-authored-by: Cursor <cursoragent@cursor.com>

platform/internal/logic/applogic/yuheng_ticket.go

@@ -83,6 +83,12 @@ func (l *AuthLogic) ExchangeYuhengTicket(req YuhengTicketExchangeReq) (*YuhengTi
 	if err != nil {
 		return nil, err
 	}
+	// Z12h：换票时顺带自愈本租户挂死通道的 Binding/Agent
+	if _, hErr := HealTenantSyncBind(l.ctx, l.svcCtx, u.TenantID); hErr == nil {
+		if refreshed, gErr := l.svcCtx.Agents.Get(l.ctx, acc.TenantID, acc.AgentID); gErr == nil && refreshed != nil {
+			acc = refreshed
+		}
+	}
 	if err := l.ensureAgentSyncPerm(acc); err != nil {
 		return nil, err
 	}