Refactor reuse random instance and duplicate literal 7times

[anukalp2804]

Description

This PR fixes duplicate literals 7 times and also solve the Save and re-use this "Random" reported by the sonarcloud analysis tool.These changes ensure the code follows the project's formatting guidelines.

Changes:

• replace the duplicate literals with constant
• creates a single reusable Random instance as a class-level constant.

Files modified:

• example/mqtt/src/main/java/org/apache/iotdb/mqtt/MQTTClient.java.

This change addresses a Sonar-reported duplicate literals for the show tables:

• https://sonarcloud.io/project/issues?open=AZUoGM3z1Lhwncm5GF-A&id=apache_iotdb
• https://sonarcloud.io/project/issues?open=AZUoGM3z1Lhwncm5GF-D&id=apache_iotdb

This PR has:

• been self-reviewed.
• been built locally with mvn spotless:apply.
• been built locally with mvn clean package -DskipTests.
• been built locally with mvn -pl iotdb-core -am test -DskipTests.