Fix: replace duplicated string literal '/myTopic' with constant

[A0R0P0I7T]

Description

This PR fixes a SonarCloud issue related to duplicated string literals in the MQTT example module.

The string literal "/myTopic" was used multiple times. It has been replaced with a constant to improve maintainability and readability.

Changes

• Introduced a constant MY_TOPIC
• Replaced all occurrences of "/myTopic" with the constant

Motivation

Fix SonarCloud issue:

• java:S1192 (duplicated string literals)

SonarCloud link:

• https://sonarcloud.io/project/issues?issueStatuses=OPEN%2CCONFIRMED&id=apache_iotdb&open=AZUoGM3z1Lhwncm5GF-A&tab=code

Verification

• Ran mvn spotless:apply
• Ran mvn clean install -DskipTests
• Build passes locally

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Checklist

• been self-reviewed
• follows project coding style (spotless applied)

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Files Modified

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