Fallback to admin token for project clusters only

We do not want group level clusters to fall back to what was old
behaviour for project level clusters. So instead we will not return any
KUBE_TOKEN if we cannot find a suitable kubernetes_namespace for the
project, in the group level cluster case.

Add test cases to assert above

app/models/clusters/platforms/kubernetes.rb

@@ -85,7 +85,7 @@ module Clusters
 
           if kubernetes_namespace = cluster.kubernetes_namespaces.has_service_account_token.find_by(project: project)
             variables.concat(kubernetes_namespace.predefined_variables)
-          else
+          elsif cluster.project_type?
             # From 11.5, every Clusters::Project should have at least one
             # Clusters::KubernetesNamespace, so once migration has been completed,
             # this 'else' branch will be removed. For more information, please see

changelogs/unreleased/legacy_fallback_for_project_clusters_only.yml

@@ -0,0 +1,5 @@
+---
+title: Fallback to admin KUBE_TOKEN for project clusters only
+merge_request: 23527
+author:
+type: other

spec/factories/clusters/kubernetes_namespaces.rb

@@ -5,10 +5,12 @@ FactoryBot.define do
     association :cluster, :project, :provided_by_gcp
 
     after(:build) do |kubernetes_namespace|
-      cluster_project = kubernetes_namespace.cluster.cluster_project
+      if kubernetes_namespace.cluster.project_type?
+        cluster_project = kubernetes_namespace.cluster.cluster_project
 
-      kubernetes_namespace.project = cluster_project.project
-      kubernetes_namespace.cluster_project = cluster_project
+        kubernetes_namespace.project = cluster_project.project
+        kubernetes_namespace.cluster_project = cluster_project
+      end
     end
 
     trait :with_token do

spec/models/clusters/platforms/kubernetes_spec.rb

@@ -273,6 +273,36 @@ describe Clusters::Platforms::Kubernetes, :use_clean_rails_memory_store_caching
         )
       end
     end
+
+    context 'group level cluster' do
+      let!(:cluster) { create(:cluster, :group, platform_kubernetes: kubernetes) }
+
+      let(:project) { create(:project, group: cluster.group) }
+
+      subject { kubernetes.predefined_variables(project: project) }
+
+      context 'no kubernetes namespace for the project' do
+        it_behaves_like 'setting variables'
+
+        it 'does not return KUBE_TOKEN' do
+          expect(subject).not_to include(
+            { key: 'KUBE_TOKEN', value: kubernetes.token, public: false }
+          )
+        end
+      end
+
+      context 'kubernetes namespace exists for the project' do
+        let!(:kubernetes_namespace) { create(:cluster_kubernetes_namespace, :with_token, cluster: cluster, project: project) }
+
+        it_behaves_like 'setting variables'
+
+        it 'sets KUBE_TOKEN' do
+          expect(subject).to include(
+            { key: 'KUBE_TOKEN', value: kubernetes_namespace.service_account_token, public: false }
+          )
+        end
+      end
+    end
   end
 
   describe '#terminals' do