Gracefully handle CI lint errors in artifacts section

If an `artifacts` section is included in the CI lint as an array instead
of a Hash, users would see 500 errors when linting the file. Even though
we separate validation that checks the type, this validation doesn't
halt other validations from running. Since `expose_as_present?` is
called inside a validation, we need to perform a redundant check to
ensure the right type.

Closes https://gitlab.com/gitlab-org/gitlab/issues/194069

changelogs/unreleased/sh-fix-ci-lint-errors.yml

+---
+title: Gracefully error handle CI lint errors in artifacts section
+merge_request: 22388
+author:
+type: fixed

lib/gitlab/ci/config/entry/artifacts.rb

@@ -54,6 +54,11 @@ module Gitlab
           def expose_as_present?
             return false unless Feature.enabled?(:ci_expose_arbitrary_artifacts_in_mr, default_enabled: true)
 
+            # This duplicates the `validates :config, type: Hash` above,
+            # but Validatable currently doesn't halt the validation
+            # chain if it encounters a validation error.
+            return false unless @config.is_a?(Hash)
+
             !@config[:expose_as].nil?
           end
         end

spec/lib/gitlab/ci/yaml_processor_spec.rb

@@ -1270,6 +1270,19 @@ module Gitlab
             expect(builds.first[:options][:artifacts][:when]).to eq(when_state)
           end
         end
+
+        it "gracefully handles errors in artifacts type" do
+          config = <<~YAML
+          test:
+            script:
+              - echo "Hello world"
+            artifacts:
+              - paths:
+              - test/
+          YAML
+
+          expect { described_class.new(config) }.to raise_error(described_class::ValidationError)
+        end
       end
 
       describe '#environment' do