Add paidTierTrial to subscriptionUsage GraphQL API

What does this MR do and why?

Related to https://gitlab.com/gitlab-org/customers-gitlab-com/-/work_items/15815

This MR exposes the paid tier trial state from CustomersDot to support the UI showing a banner with link to CustomersDot.

This adds a new PaidTierTrialType GraphQL type with an isActive boolean field to the subscriptionUsage schema. The field delegates to CustomersDot and maps paidTierTrial.isActive from the metadata response.

Changes:

• Extend CDOT metadata query to request paidTierTrial { isActive }
• Add PaidTierTrialType GraphQL type with isActive field
• Add paid_tier_trial method to SubscriptionUsage model
• Update SubscriptionUsageType to include paid_tier_trial field

References

https://gitlab.com/gitlab-org/customers-gitlab-com/-/work_items/15815+

Screenshots or screen recordings

How to set up and validate locally

1. Login to your GDK at http://localhost:3000

2. Pick a group you want to test with and note the namespace path

3. Go to http://localhost:3000/-/graphql-explorer

4. Enter the query as shown below:

    query getSubscriptionUsage($namespacePath: ID) {
      subscriptionUsage(namespacePath: $namespacePath) {
        startDate
        endDate
        paidTierTrial {
          isActive
        }
      }
    }

   With variables:

   {
     "namespacePath": "clickhouse"
   }

5. The response should contain the new paidTierTrial.isActive field.

MR acceptance checklist

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