Use `AgentTokensFinder` to find single agent tokens

app/finders/clusters/agent_tokens_finder.rb

@@ -2,6 +2,8 @@
 
 module Clusters
   class AgentTokensFinder
+    include FinderMethods
+
     def initialize(agent, current_user)
       @agent = agent
       @current_user = current_user

app/models/clusters/agent_token.rb

@@ -25,5 +25,9 @@ class AgentToken < ApplicationRecord
       active: 0,
       revoked: 1
     }
+
+    def to_ability_name
+      :cluster
+    end
   end
 end

lib/api/clusters/agent_tokens.rb

@@ -43,8 +43,7 @@ class AgentTokens < ::API::Base
             end
             get ':token_id' do
               agent = ::Clusters::AgentsFinder.new(user_project, current_user).find(params[:agent_id])
-
-              token = agent.agent_tokens.find(params[:token_id])
+              token = ::Clusters::AgentTokensFinder.new(agent, current_user).find(params[:token_id])
 
               present token, with: Entities::Clusters::AgentToken
             end

spec/models/clusters/agent_token_spec.rb

@@ -49,4 +49,12 @@
       expect(agent_token.token.length).to be >= 50
     end
   end
+
+  describe '#to_ability_name' do
+    it 'is :cluster' do
+      agent_token = build(:cluster_agent_token)
+
+      expect(agent_token.to_ability_name).to eq :cluster
+    end
+  end
 end

spec/requests/api/clusters/agent_tokens_spec.rb

@@ -80,6 +80,27 @@
         end
       end
 
+      it 'returns an agent token that is revoked' do
+        get api("/projects/#{project.id}/cluster_agents/#{agent.id}/tokens/#{revoked_agent_token.id}", user)
+
+        aggregate_failures "testing response" do
+          expect(response).to have_gitlab_http_status(:ok)
+          expect(response).to match_response_schema('public_api/v4/agent_token')
+          expect(json_response['id']).to eq(revoked_agent_token.id)
+          expect(json_response['name']).to eq(revoked_agent_token.name)
+          expect(json_response['agent_id']).to eq(agent.id)
+          expect(json_response['status']).to eq('revoked')
+        end
+      end
+
+      it 'returns a 404 if agent does not exist' do
+        path = "/projects/#{project.id}/cluster_agents/#{non_existing_record_id}/tokens/#{non_existing_record_id}"
+
+        get api(path, user)
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+
       it 'returns a 404 error if agent token id is not available' do
         get api("/projects/#{project.id}/cluster_agents/#{agent.id}/tokens/#{non_existing_record_id}", user)