service hook: get correct NS for task-level consul (#19242)

Ensure that the `ServiceProviderNamespace` correctly picks the task-level
`consul.namespace` and falls back to the group if set.

nomad/structs/alloc.go

@@ -53,7 +53,7 @@ func (a *Allocation) ServiceProviderNamespace() string {
 			case ServiceProviderNomad:
 				return a.Job.Namespace
 			default:
-				return tg.Consul.GetNamespace()
+				return a.ConsulNamespaceForTask(task.Name)
 			}
 		}
 	}