[fix][test] Fix flaky BrokerRegistryIntegrationTest port binding race

[merlimat]

Motivation

BrokerRegistryIntegrationTest.setup is flaky due to a port binding race condition. Ports are allocated at field initialization time via PortManager.nextFreePort(), but by the time @BeforeClass runs, those ports may have been reused by another test class.

Stack trace

Gradle suite > Gradle test > BrokerRegistryIntegrationTest > setup FAILED
    java.io.IOException: java.net.BindException: Address already in use
        at LocalBookkeeperEnsemble.runZookeeper(LocalBookkeeperEnsemble.java:219)
        at LocalBookkeeperEnsemble.start(LocalBookkeeperEnsemble.java:420)
        at BrokerRegistryIntegrationTest.setup(BrokerRegistryIntegrationTest.java:52)
        Caused by: java.net.BindException: Address already in use

BrokerRegistryIntegrationTest.cleanup FAILED
    java.lang.NullPointerException at BrokerRegistryIntegrationTest.java:71

Modifications

• Move LocalBookkeeperEnsemble creation from field initialization to @BeforeClass
• Use port 0 for automatic OS port allocation instead of PortManager.nextFreePort()
• Add null guard for bk in cleanup to handle setup failures gracefully

Documentation

• doc-not-needed