Fix offline CUSTOMIZED evacuation check to use union instead of intersection

[LZD-PratyushBhatt]

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Fix offline CUSTOMIZED evacuation check to use union instead of intersection
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For offline instances, treat blocking partitions as the union of CurrentState partitions and IdealState assignments, instead of only their intersection. This prevents false evacuation completion when IdealState partition names rotate but the instance still has CurrentState entries.

Context: this is the behavior that pinot desires. They want union and not the intersection(current behavior). Because of this behavior one of their instance was prematurely deleted.

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