Fix add-on purchase lookup in GitLab Duo setup script

What does this MR do and why?

Fixes support/setup/gitlab_duo_setup.rb, which now fails on GitLab master with:

PG::UndefinedColumn: ERROR:  column subscription_add_on_purchases.add_on does not exist

gitlab!258275 (merged) changed GitlabSubscriptions::AddOnPurchase#add_on to a belongs_to_fixed_items association. ActiveRecord no longer sees it as an association, so AddOnPurchase.find_by(add_on: ...) becomes a lookup on a column that doesn't exist. This MR looks the purchase up through the for_duo_enterprise and for_gitlab_duo_pro scopes instead. Both scopes exist in older GitLab versions too, so update-from-previous scenarios keep working.

This breaks every scenario that seeds Duo seats, for example the duo-agent-platform job added to gitlab-org/gitlab in gitlab!264661 (merged):

• https://gitlab.com/gitlab-org/gitlab/-/jobs/17468561410
• https://gitlab.com/gitlab-org/gitlab/-/jobs/17470109995

After this is merged, we need a gitlab-qa release and a version bump in gitlab-org/gitlab's qa/Gemfile.

Screenshots or screen recordings

N/A

How to set up and validate locally

1. Against a GitLab instance with a Duo Enterprise or Duo Pro add-on purchase, run:

   ASSIGN_SEATS=true HAS_ADD_ON=true QA_EE_ACTIVATION_CODE=<code> gitlab-rails runner support/setup/gitlab_duo_setup.rb

2. Confirm it prints Seat assignment for admin successful.

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