feat(validator): add Kubeflow Trainer support to robust-controller check

The robust-controller conformance check previously only validated the
Dynamo operator, causing it to skip on all training clusters. This adds
Kubeflow Trainer as an alternative target, selected based on recipe
component presence:

- dynamo-platform in recipe → validate Dynamo operator
- kubeflow-trainer in recipe → validate Kubeflow Trainer
- neither → skip

Kubeflow Trainer validation checks:
1. Controller deployment running (kubeflow-trainer-controller-manager)
2. Validating webhook operational with reachable endpoint
3. TrainJob CRD exists (trainjobs.trainer.kubeflow.org)
4. Webhook rejects invalid TrainJob (behavioral test)

Refactored the original Dynamo validation into checkRobustDynamo() and
renamed validateWebhookRejects to validateDynamoWebhookRejects for
clarity.

Author: yuanchen8911

recipes/validators/catalog.yaml

@@ -122,7 +122,7 @@ validators:
     env: []
   - name: robust-controller
     phase: conformance
-    description: "Verify Dynamo operator controller and webhooks"
+    description: "Verify AI operator controller and webhooks (Dynamo or Kubeflow Trainer)"
     image: ghcr.io/nvidia/aicr-validators/conformance:latest
     timeout: 5m
     args: ["robust-controller"]