Add default slack-hook params to fix the flux kustomize-controller errors

Description / Motivation: The kustomize-controller is failing with

```
kubectl logs kustomize-controller-8589b7fd57-62kv4 -n flux-system | grep error
...
{"level":"error","ts":"2025-10-28T00:20:58.625Z","logger":"controller.kustomization","msg":"Reconciliation failed after 482.930394ms, next try in 2m0s","reconciler group":"kustomize.toolkit.fluxcd.io","reconciler kind":"Kustomization","name":"kubernetes-iteration-toolkit","namespace":"tekton-pipelines","revision":"main/5009ee34941fc5f77054986fc450c5be68cc8bb0","error":"Pipeline/scalability/derekff-karpenter-testing dry-run failed, reason: BadRequest, error: admission webhook \"validation.webhook.pipeline.tekton.dev\" denied the request: validation failed: non-existent variable in \"$(params.slack-hook)\": spec.finally[0].params[slack-hook]\n"}
```

This is likely blocking Flux to sync from KIT.

Author: chithresha

tests/tekton-resources/pipelines/eks/karpenter-ultra.yaml

@@ -80,6 +80,8 @@ spec:
   - default: ""
     name: aws-account-id
     type: string
+  - default: ""
+    name: slack-hook
   tasks:
   - name: awscli-vpc-create
     params: