The following methods should be declared const as they do not modify any state of the queried objects:

• PlanStage::isEOF()
• PlanExecutor::isEOF()
• PlanExecutor::getKillStatus()

This should not change behavior or performance in any way, but strengthens the API contracts of these methods. Callers would not expect these methods to modify the state anyway. By marking them as explicitly const, we can prevent them from unintentionally modifying state in the future.