Keep isSSL() and getClientAddr() package-private for test accessibility

Methods stubbed via Mockito spy in same-package tests must remain
package-private. Revert isSSL() to package-private; getClientAddr()
follows the same convention as getAddress().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -180,7 +180,7 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
         this.rpcAwaitingAck = new ConcurrentHashMap<>();
     }
 
-    private boolean isSSL() {
+    boolean isSSL() {
         return sslHandler != null;
     }
 
@@ -258,7 +258,7 @@ public class MqttTransportHandler extends ChannelInboundHandlerAdapter implement
         }
     }
 
-    private String getClientAddr(ChannelHandlerContext ctx) {
+    String getClientAddr(ChannelHandlerContext ctx) {
         try {
             InetSocketAddress remote = getAddress(ctx);
             if (remote == null) return "unknown";