Fix NullPointerException in WAGED rebalancer when instance config is deleted

[LZD-PratyushBhatt]

Issues

• My PR addresses the following Helix issues and references them in the PR description:
  Fixes a NullPointerException in WagedInstanceCapacity that occurs when an instance config is deleted while partitions are still assigned to it. The exception prevents the controller pipeline from completing, blocking rebalancing and leaving stale assignments in IdealState.

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Description

When an instance config is deleted:

1. The instance is removed from _instanceCapacityMap during initialization
2. CurrentState may still have partitions assigned to that instance
3. checkAndReduceInstanceCapacity() access instanceCapacity.keySet() on a null value
4. This causes a NullPointerException that crashes the DEFAULT pipeline
5. The rebalancer never runs, so the deleted instance remains in IdealState assignments

Added null checks in methods to handle deleted instance configs:

• checkAndReduceInstanceCapacity(): Returns false and logs when instance config is missing
  This allows the pipeline to continue, and the rebalancer removes the deleted instance from assignments in the next cycle.

• Here are some details about my PR, including screenshots of any UI changes:

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Tests

• Reproduced the issue locally by deleting an instance config while it had partition assignments

• Verified the fix prevents the NPE and allows rebalancing to proceed

• Confirmed the deleted instance is removed from IdealState after rebalancing

• The following tests are written for this issue:

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• The following is the result of the "mvn test" command on the appropriate module:

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Changes that Break Backward Compatibility (Optional)

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Documentation (Optional)

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Commits

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Code Quality

• My diff has been formatted using helix-style.xml
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[kabragaurav]

LGTM