Fix global lock contention in DistClusterControllerStateModel caused by Optional.empty() singleton

[LZD-PratyushBhatt]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  This PR fixes _controllerOpt getting shared across resources apache/helix#3050 : _controllerOpt getting shared across resources

Description

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  Apache Helix PR: Fix global lock contention in DistClusterControllerStateModel caused by Optional.empty() singleton apache/helix#3052

Refer to apache#3050 for Problem description.
Basically, all DistClusterControllerStateModel instances were synchronizing on the same global lock object due to Optional.empty() returning a singleton instance.
Replaced synchronization on _controllerOpt (the shared singleton) with a dedicated per-instance lock object _controllerLock = new Object(). This eliminates cross-instance contention while preserving within-instance synchronization guarantees.
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Tests

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  Added a new test testNoSharedLockAcrossInstances
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Code Quality

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[proud-parselmouth]

LGTM. Thanks for driving this.