fix: release DirectByteBuffer after MQTT SSL auth to prevent native memory leak

Holding sslHandler as a final field kept SSLEngine and its JNI-allocated
DirectByteBuffers alive for the full session lifetime. On rapid reconnect
loops this caused linear native memory growth, eventually hitting OOM.

Null the reference at the end of onValidateDeviceResponse() so SslHandler
becomes eligible for Young GC immediately after authentication completes.
Add boolean ssl flag to preserve isSSL semantics after the field is cleared.

common/transport/mqtt/src/main/java/org/thingsboard/server/transport/mqtt/MqttTransportHandler.java

@@ -150,7 +150,8 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
     protected final MqttTransportContext context;
     private final TransportService transportService;
     private final SchedulerComponent scheduler;
-    private final SslHandler sslHandler;
+    private final boolean ssl;
+    private volatile SslHandler sslHandler;
     private final ConcurrentMap<MqttTopicMatcher, Integer> mqttQoSMap;
 
     final DeviceSessionCtx deviceSessionCtx;
@@ -172,6 +173,7 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
         this.context = context;
         this.transportService = context.getTransportService();
         this.scheduler = context.getScheduler();
+        this.ssl = sslHandler != null;
         this.sslHandler = sslHandler;
         this.mqttQoSMap = new ConcurrentHashMap<>();
         this.deviceSessionCtx = new DeviceSessionCtx(sessionId, mqttQoSMap, context);
@@ -1373,6 +1375,7 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
                 }
             });
         }
+        this.sslHandler = null;
     }
 
     @Override