fix(): Missing cluster silently skips gateway creation, leaving slice with no inter-cluster connectivity

[Ady0333]

Description

createMinimumGatewaysIfNotExists() queries each cluster referenced in SliceConfig.Spec.Clusters
to build the gateway mesh. When a Cluster CR is absent, util.GetResourceIfExist() returns
(false, nil). The original code checked if !found || err != nil { return ctrl.Result{}, err } —
when !found==true and err==nil, this returns nil to the work queue.

Before (broken):

found, err := util.GetResourceIfExist(ctx, client.ObjectKey{...}, &cluster)
if !found || err != nil {
    return ctrl.Result{}, err  // returns nil when cluster missing
}

The SliceConfig reconcile sees nil, treats it as success, and drops from the work queue. Meanwhile,
the earlier CreateMinimalWorkerSliceConfig step already created WorkerSliceConfig objects for all
clusters (it doesn't validate cluster existence). The slice is now in a ghost state: worker configs
exist, but the gateway mesh is never built zero cross-cluster connectivity.

  After (fixed):
  if err != nil {
      return ctrl.Result{}, err
  }
  if !found {
      return ctrl.Result{}, fmt.Errorf("cluster %s not found in namespace %s", clusterName, namespace)
  }                                                                                                     
  Now a missing cluster explicitly fails the reconcile, causing the work queue to requeue with
  exponential backoff until the Cluster CR is registered.

How Has This Been Tested?

• Wrote a unit test (Test_ClusterNotFound_GatewayCreationSilentlySkipped) that mocks a missing
  Cluster CR during gateway creation
• Before fix: function returns nil error ghost slice confirmed
• After fix: function returns error "cluster ... not found" -- SliceConfig will be requeued

Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have tested it for all user roles.
• I have added all the required unit test cases.

Does this PR introduce a breaking change for other components like worker-operator?

No breaking changes. The fix makes error handling stricter only affects behavior when a referenced Cluster is missing (previously silent, now properly requeued).
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