A heartbreaking 96% of pleas to help homeless New Yorkers never actually led to any assistance since the deadly cold snap hit the Big Apple  — with city workers unable to even find the destitute denizens most of the time, The Post has learned. The stunning struggles revealed by 311 data came as the city’s medical examiner testified Tuesday that at least 15 of 18 deaths during the deep freeze were directly related to hypothermia. City Council members grilled outgoing Department of Social