Backport of 'update the active_add_on_purchase check to include self-managed check'

What does this MR do and why?

backport !200258 (merged) to 18-2

#557958 (comment 2682743101)

for self-managed instance the namespace id in AddOnPurchase table is nil which caused the check fail:

::GitlabSubscriptions::AddOnPurchase
        .joins(:add_on)
        .where(
          namespace_id: self_and_ancestor_ids,
          subscription_add_ons: { name: GitlabSubscriptions::AddOn.names[add_on] }
        )
        .active
        .any?

Thus the self-managed customers can't use duo code review.

in this MR I added the check for self-managed instance

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.

Screenshots or screen recordings

Before	After

With a valid AddOnPurchase record where the namespace_id is nil,

 #<GitlabSubscriptions::AddOnPurchase:0x00000001652fb690
  id: 1,
  created_at: Tue, 22 Apr 2025 19:00:00.917180000 UTC +00:00,
  updated_at: Fri, 22 Aug 2025 19:00:19.410037000 UTC +00:00,
  subscription_add_on_id: 3,
  namespace_id: nil,
  quantity: 100,
  expires_on: Sat, 22 Aug 2026,
  purchase_xid: "C-00769226",
  last_assigned_users_refreshed_at: Fri, 22 Aug 2025 00:00:13.701873000 UTC +00:00,
  trial: false,
  started_at: Tue, 10 Sep 2024,
  organization_id: 1>]

in a self-managed instance:

[19] pry(main)> ::Gitlab::Saas.feature_available?(:gitlab_com_subscriptions)
=> false

We can see the duo review available: