Deleting a waypoint service can panic the workload controller

[avinxshKD]

What happened:

waypointCache.DeleteService() removes the waypoint entry but leaves references to it in serviceToWaypoint and workloadToWaypoint.

If one of those resources is updated afterwards, the controller calls deleteService() or deleteWorkload() on a missing map entry and panics.

What you expected to happen:

Deleting a waypoint should clean up its associations without crashing on later updates.

How to reproduce it (as minimally and precisely as possible):

This unit test reproduces it:

func TestDeleteWaypointBeforeAssociatedService(t *testing.T) {
	serviceCache := NewServiceCache()
	cache := NewWaypointCache(serviceCache)

	waypoint := "default/waypoint.default.svc.cluster.local"
	svc := common.CreateFakeService("app", "10.240.10.1", waypoint, nil)

	cache.AddOrUpdateService(svc)
	cache.DeleteService(waypoint)

	svc.Waypoint = nil
	cache.AddOrUpdateService(svc) // nil pointer panic
}

The same state can happen when the waypoint service is removed before updates for its associated services or workloads arrive.

Anything else we need to know?:
DeleteService() guards the lookup when removing an associated service, but the add/update paths dereference it directly.

Environment:
Kmesh version: main (43d5f93)
Kmesh mode: Dual-Engine Mode
Istio version: N/A
Kernel version: N/A
Others: N/A

[avinxshKD]

/assign