Gracefully handle unknown failure reason in runner CI job

Relates to
https://gitlab.com/gitlab-com/gl-infra/production/-/issues/4869

Changelog: fixed

app/services/ci/update_build_state_service.rb

@@ -169,7 +169,7 @@ module Ci
         state: params.fetch(:state),
         trace_checksum: trace_checksum,
         trace_bytesize: trace_bytesize,
-        failure_reason: params.dig(:failure_reason)
+        failure_reason: failure_reason
       )
 
       unless build_state.present?
@@ -179,6 +179,14 @@ module Ci
       build_state || build.pending_state
     end
 
+    def failure_reason
+      reason = params.dig(:failure_reason)
+
+      return unless reason
+
+      Ci::BuildPendingState.failure_reasons.fetch(reason.to_s, 'unknown_failure')
+    end
+
     ##
     # This method is releasing an exclusive lock on a build trace the moment we
     # conclude that build status has been written and the build state update

spec/services/ci/update_build_state_service_spec.rb

@@ -15,6 +15,24 @@ RSpec.describe Ci::UpdateBuildStateService do
     stub_feature_flags(ci_enable_live_trace: true)
   end
 
+  context 'when build has unknown failure reason' do
+    let(:params) do
+      {
+        output: { checksum: 'crc32:12345678', bytesize: 123 },
+        state: 'failed',
+        failure_reason: 'no idea here',
+        exit_code: 42
+      }
+    end
+
+    it 'updates a build status' do
+      result = subject.execute
+
+      expect(build).to be_failed
+      expect(result.status).to eq 200
+    end
+  end
+
   context 'when build does not have checksum' do
     context 'when state has changed' do
       let(:params) { { state: 'success' } }