feat(diagnosis): add protocol-aware connectivity diagnosis for tunnel chains

- Pass tunnel protocol through diagnosis work items
- Support protocol-specific ping (TCP/UDP/KCP) via remote nodes
- Add KCP probe support in websocket_reporter for chain hop testing

go-backend/internal/store/model/model.go

@@ -546,6 +546,7 @@ type TunnelRecord struct {
 	Status       int
 	Flow         int64
 	TrafficRatio float64
+	Protocol     string
 }
 
 type UserQuotaView struct {