Use the correct project path in generated `KUBECONFIG` file

Previously an Agent had to be directly associated with a project
for it to be used for the CI tunnel, which meant that the Agent
configuration project was the same project that was running the build.

With the introduction of authorizations for arbitrary groups and
projects, this assumption no longer holds. We must instead use the
project associated with the Agent instead of the one associated with
the build.

https://gitlab.com/gitlab-org/gitlab/-/merge_requests/70452

Changelog: fixed
EE: true

ee/app/services/ci/generate_kubeconfig_service.rb

@@ -4,7 +4,6 @@ module Ci
   class GenerateKubeconfigService
     def initialize(build)
       @build = build
-      @project = build.project
       @template = Gitlab::Kubernetes::Kubeconfig::Template.new
     end
 
@@ -34,7 +33,7 @@ module Ci
 
     private
 
-    attr_reader :build, :project, :template
+    attr_reader :build, :template
 
     def agents
       build.pipeline.authorized_cluster_agents
@@ -49,7 +48,7 @@ module Ci
     end
 
     def context_name(agent)
-      [project.full_path, agent.name].join(delimiter)
+      [agent.project.full_path, agent.name].join(delimiter)
     end
 
     def agent_token(agent)

ee/spec/services/ci/generate_kubeconfig_service_spec.rb

@@ -7,7 +7,7 @@ RSpec.describe Ci::GenerateKubeconfigService do
     let(:project) { create(:project) }
     let(:build) { create(:ci_build, project: project) }
     let(:agent1) { create(:cluster_agent, project: project) }
-    let(:agent2) { create(:cluster_agent, project: project) }
+    let(:agent2) { create(:cluster_agent) }
 
     let(:template) { instance_double(Gitlab::Kubernetes::Kubeconfig::Template) }
 
@@ -39,7 +39,7 @@ RSpec.describe Ci::GenerateKubeconfigService do
         user: "agent:#{agent1.id}"
       )
       expect(template).to receive(:add_context).with(
-        name: "#{project.full_path}:#{agent2.name}",
+        name: "#{agent2.project.full_path}:#{agent2.name}",
         cluster: 'gitlab',
         user: "agent:#{agent2.id}"
       )