Prevent 500 on parallel org quota creation

To apply this change the migration from
#3923 needs to
be in first. Tha-s why we ceeated the adoption to prevetn 500 on
concurrent requests for organization_quota_create separately. The reason
is the same as in #3899 and #3918.

app/models/runtime/quota_definition.rb

@@ -19,6 +19,15 @@ class QuotaDefinition < Sequel::Model
                       :app_instance_limit, :app_task_limit, :total_service_keys, :total_reserved_route_ports,
                       :log_rate_limit
 
+    def around_save
+      yield
+    rescue Sequel::UniqueConstraintViolation => e
+      raise e unless e.message.include?('qd_name_index')
+
+      errors.add(:name, :unique)
+      raise validation_failed_error
+    end
+
     def validate
       validates_presence :name
       validates_unique :name

spec/unit/actions/organization_quotas_create_spec.rb

@@ -132,6 +132,26 @@ module VCAP::CloudController
           end
         end
 
+        context 'when creating organization quotas concurrently' do
+          let(:name) { 'awesome' }
+          let(:message) { VCAP::CloudController::OrganizationQuotasCreateMessage.new(name:) }
+
+          it 'ensures one creation is successful and the other fails due to name conflict' do
+            # First request, should succeed
+            expect do
+              org_quotas_create.create(message)
+            end.not_to raise_error
+
+            # Mock the validation for the second request to simulate the race condition and trigger a unique constraint violation
+            allow_any_instance_of(QuotaDefinition).to receive(:validate).and_return(true)
+
+            # Second request, should fail with correct error
+            expect do
+              org_quotas_create.create(message)
+            end.to raise_error(OrganizationQuotasCreate::Error, "Organization Quota 'awesome' already exists.")
+          end
+        end
+
         context 'when the org guid is invalid' do
           let(:invalid_org_guid) { 'invalid_org_guid' }
           let(:message_with_invalid_org_guid) do