Fix wrong value of checkbox between Rails / Vue

When a checkbox field is saved in the integration form,
in Rails (without integration_form_refactor ff) it gets saved
as "1" for true and "0" for false, while in Vue we use "true"
and "false". This caused a 1-time transition mismatch
between the values of checkboxes when the feature flag
was enabled.

app/serializers/service_field_entity.rb

@@ -11,6 +11,8 @@ class ServiceFieldEntity < Grape::Entity
 
     if field[:type] == 'password' && value.present?
       'true'
+    elsif field[:type] == 'checkbox'
+      ActiveRecord::Type::Boolean.new.deserialize(value).to_s
     else
       value
     end

changelogs/unreleased/227363-project-services-checkboxes-are-not-checked-with-feature-flag-inte.yml

+---
+title: Fix wrong value of checkbox in integration form
+merge_request: 36329
+author:
+type: fixed

spec/serializers/service_field_entity_spec.rb

@@ -68,7 +68,7 @@ RSpec.describe ServiceFieldEntity do
             placeholder: nil,
             required: nil,
             choices: nil,
-            value: true
+            value: 'true'
           }
 
           is_expected.to include(expected_hash)