refactor(grpc/client): simplify processEventStream with per-event handlers

- Extract eventHandlers dispatch table
- Add per-event handlers: handleSlugChange, handleGetSessions, handleTerminateSession
- Introduce sendNode helper to centralize send/error handling and preserve connection-error propagation
- Add protoToTunnelType for tunnel-type validation
- Map unknown proto.TunnelType to types.UNKNOWN in protoToTunnelType and return a descriptive error
- Reduce boilerplate and improve readability of processEventStream

types/types.go

@@ -11,8 +11,9 @@ const (
 type TunnelType string
 
 const (
-	HTTP TunnelType = "HTTP"
-	TCP  TunnelType = "TCP"
+	UNKNOWN TunnelType = "UNKNOWN"
+	HTTP    TunnelType = "HTTP"
+	TCP     TunnelType = "TCP"
 )
 
 type SessionKey struct {