Fix: reset assigned_host to empty string to avoid NoneType IP during kernel restart (#142)

(We'll address the unrelated test failures another day.)

Author: Recai93

gateway_provisioners/yarn.py

@@ -240,8 +240,9 @@ async def cleanup(self, restart: bool = False) -> None:
             self.local_proc.wait()
             self.local_proc = None
 
-        # reset application id to force new query - handles kernel restarts/interrupts
+        # reset application id and assigned host to force new query - handles kernel restarts/interrupts
         self.application_id = None
+        self.assigned_host = ""
 
         # for cleanup, we should call the superclass last
         await super().cleanup(restart=restart)