Expose `failure_reasons` in `Build#features`

This makes us to expose a list of failure
reasons available for in a context of a given
build allowing Runner to sanitise what is being
sent.

app/models/ci/build.rb

@@ -564,7 +564,10 @@ module Ci
     end
 
     def features
-      { trace_sections: true }
+      {
+        trace_sections: true,
+        failure_reasons: self.class.failure_reasons.keys
+      }
     end
 
     def merge_request

changelogs/unreleased/expose-failure-reasons-features.yml

+---
+title: Expose `failure_reasons` in `Build#features`
+merge_request:
+author:
+type: added

spec/requests/api/ci/runner/jobs_request_post_spec.rb

@@ -198,7 +198,12 @@ RSpec.describe API::Ci::Runner, :clean_gitlab_redis_shared_state do
                'when' => 'on_success' }]
           end
 
-          let(:expected_features) { { 'trace_sections' => true } }
+          let(:expected_features) do
+            {
+              'trace_sections' => true,
+              'failure_reasons' => include('script_failure')
+            }
+          end
 
           it 'picks a job' do
             request_job info: { platform: :darwin }
@@ -220,7 +225,7 @@ RSpec.describe API::Ci::Runner, :clean_gitlab_redis_shared_state do
             expect(json_response['artifacts']).to eq(expected_artifacts)
             expect(json_response['cache']).to eq(expected_cache)
             expect(json_response['variables']).to include(*expected_variables)
-            expect(json_response['features']).to eq(expected_features)
+            expect(json_response['features']).to match(expected_features)
           end
 
           it 'creates persistent ref' do