Return client error for duplicate Terraform state serials

Changelog: fixed

Fixes https://gitlab.com/gitlab-org/gitlab/-/issues/344085

lib/api/terraform/state.rb

@@ -11,6 +11,13 @@ module API
 
       default_format :json
 
+      rescue_from(
+        ::ActiveRecord::RecordNotUnique,
+        ::PG::UniqueViolation
+      ) do |e|
+        render_api_error!(e.message, 422)
+      end
+
       before do
         authenticate!
         authorize! :read_terraform_state, user_project

spec/requests/api/terraform/state_spec.rb

@@ -152,6 +152,16 @@ RSpec.describe API::Terraform::State do
           expect(response).to have_gitlab_http_status(:ok)
           expect(Gitlab::Json.parse(response.body)).to be_empty
         end
+
+        context 'when serial already exists' do
+          let(:params) { { 'instance': 'example-instance', 'serial': state.latest_version.version } }
+
+          it 'returns unprocessable entity' do
+            request
+
+            expect(response).to have_gitlab_http_status(:unprocessable_entity)
+          end
+        end
       end
 
       context 'without body' do