[Draft] Fix cleanup of GRPCRoute resources in namespaces without psm-csm suffix

[sreenithi]

[DO-NOT-MERGE] Yet to be tested.

Cleanup job was recently failing for some namespaces like:

I0712 14:44:50.416528 140554289934336 k8s_base_runner.py:943] Deleting namespace psm-interop-server-20260618-1647-r9cyq
E0712 14:49:51.754359 140554289934336 cleanup.py:535] Timeout waiting for namespace psm-interop-server-20260618-1647-r9cyq deletion. Failed resource status:

Resource name: psm-interop-server-20260618-1647-r9cyq
Resource kind: Namespace
Created: 2026-06-18 16:48:05+00:00; 23 days ago
Deletion requested: 2026-06-21 15:17:41+00:00; 20 days ago

---
conditions:
- last_transition_time: 2026-07-03 21:17:18+00:00
  message: All resources successfully discovered
  reason: ResourcesDiscovered
  status: 'False'
  type: NamespaceDeletionDiscoveryFailure
- last_transition_time: 2026-06-21 15:17:46+00:00
  message: All legacy kube types successfully parsed
  reason: ParsedGroupVersions
  status: 'False'
  type: NamespaceDeletionGroupVersionParsingFailure
- last_transition_time: 2026-06-21 15:17:46+00:00
  message: All content successfully deleted, may be waiting on finalization
  reason: ContentDeleted
  status: 'False'
  type: NamespaceDeletionContentFailure
- last_transition_time: 2026-06-21 15:17:56+00:00
  message: 'Some resources are remaining: servicenetworkendpointgroups.networking.gke.io
    has 1 resource instances'
  reason: SomeResourcesRemain
  status: 'True'
  type: NamespaceContentRemaining
- last_transition_time: 2026-06-21 15:17:56+00:00
  message: 'Some content in the namespace has finalizers remaining: [networking.gke.io/neg-finalizer](https://www.google.com/url?q=http://networking.gke.io/neg-finalizer&sa=D)
    in 1 resource instances'
  reason: SomeFinalizersRemain
  status: 'True'
  type: NamespaceFinalizersRemaining
phase: Terminating
...


E0712 14:49:51.754550 140554289934336 cleanup.py:644] ----- Skipped [cleanup timed out]: psm-interop-server-20260618-1647-r9cyq

When inspecting these name spaces manually, found that these have a pending backend service and a gRPC route attached to it

$ kubectl get servicenetworkendpointgroups.networking.gke.io -n psm-interop-server-20260618-1647-r9cyq
NAME                                                     AGE
psm-interop-server-20260618-1647-r9cyq-psm-grpc-server   24d

$ gcloud compute backend-services delete psm-interop-backend-service-20260618-1647-r9cyq   --region=us-west1   --project=grpc-testing
The following region backend services will be deleted:
 - [psm-interop-backend-service-20260618-1647-r9cyq] in [us-west1]

Do you want to continue (Y/n)?  y

ERROR: (gcloud.compute.backend-services.delete) Some requests did not succeed:
 - The resource cannot be deleted because it's being used by //networkservices.googleapis.com/projects/830293263384/locations/us-west1/grpcRoutes/psm-interop-grpc-route-20260618-1647-r9cyq

We ideally create a gRPC route only in gamma tests that should have a psm-csm suffix in the namespace. So Gamma servers cleanups that are triggered for psm-csm prefixed namespaces handle the grpc-route deletion correctly.
But looks like these non-Gamma namespaces also have a gRPC route attached with them at times and don't clean them correctly.
So, this PR just checks if such a route resource exists and delegates the cleanup to the Gamma servers cleanups instead.

Note:
However, the above problematic namespaces have been manually cleaned up already, so there is currently no way to test this change. Will open this up for review once tested.