Add a confirmation alert for Duo Pro

What does this MR do and why?

Add a confirmation alert post-trial creation for Duo Pro

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

• If you don't have working CDot locally (and you probably don't):
  • Update generate_addon_trial to return { success: true } (this simulates successful Duo Pro trial creation):

          def generate_addon_trial(params)
            return { success: true }
          end

• Setup gdk to simulate SaaS - https://docs.gitlab.com/ee/development/ee_features.html#simulate-a-saas-instance and restart GDK
• Sign up. Create new group during sign up or afterwards.

bin/rails c

Feature.enable(:duo_pro_trials)
Feature.enable(:cs_connect_with_sales)
GitlabSubscriptions::AddOn.find_or_create_by(name: 'code_suggestions', description: GitlabSubscriptions::AddOn.descriptions[:code_suggestions])
GitlabSubscription.last.update(hosted_plan: Plan.find_by(name: 'ultimate'))

• Visit created group billing page. Click Start a Duo Pro trial.
• Add &step=trial to the url. Submit new form.

Related to #451852