refactor(plugins): validate scheduler capability at load time

Move scheduler capability check from runtime (when callback fires) to
load-time validation in ValidateWithCapabilities. This ensures plugins
declaring the scheduler permission must export the nd_scheduler_callback
function, failing fast with a clear error instead of silently skipping
callbacks at runtime.

plugins/host_scheduler.go

@@ -188,12 +188,6 @@ func (s *schedulerServiceImpl) invokeCallback(ctx context.Context, scheduleID st
 		return
 	}
 
-	// Check if plugin has the scheduler capability
-	if !hasCapability(instance.capabilities, CapabilityScheduler) {
-		log.Warn(ctx, "Plugin does not have scheduler capability", "plugin", s.pluginName, "scheduleID", scheduleID)
-		return
-	}
-
 	// Prepare callback input
 	input := capabilities.SchedulerCallbackRequest{
 		ScheduleID:  scheduleID,