NIFI-16180 - Ensure Rebalancing Worker Stops Before Returning

[pvillard31]

Summary

NIFI-16180 - Ensure Rebalancing Worker Stops Before Returning

This change fixes a recurring race in TestSocketLoadBalancedFlowFileQueue by ensuring the rebalancing worker has fully stopped before stop() returns. It also prevents stop and start operations from overlapping and safely returns any FlowFiles already picked up by the worker so they remain correctly counted and are redistributed once after restart.

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