Go figure: A non-Jewish, non-Zionist politician has sparked a national Jewish conversation about the role of the rabbi. If elected next week, the 34-year-old progressive Zohran Mamdani would be the first mayor of New York City who came up through the trenches of pro-Palestinian activism, and the first to reject the idea that being mayor of a city with 1 million Jews means being a supporter of Israel.The prospect has shaken a Jewish mainstream that has long taken that support for granted,