Photo: Robert Nickelsberg/Getty Images Many of the challenges Zohran Mamdani has encountered in his first 100 days as mayor — like ending street homelessness, halting the deterioration of public housing , or preventing the deaths in custody of jailed detainees on Rikers Island — are not just tough but essentially unsolvable . But raising the abysmally low wages of emergency medical-services workers, New York’s fabled corps of skilled “street doctors” who staff fire-department ambulances and