Add nonBillableUsage to GitlabSubscriptionUsage

What does this MR do and why?

Adds nonBillableUsage to GitlabSubscriptionUsage, so the GitLab Credits dashboards (group dashboard on GitLab.com, instance dashboard on self-managed) can show usage that was not billed, per product and skip reason.

It proxies CustomersDot's new gitlabCreditsUsage.nonBillableUsage field:

• Client: SubscriptionUsageClient#get_non_billable_usage sends its own query (GET_NON_BILLABLE_USAGE_QUERY), so a failure here does not fail creditsUsed / dailyUsage. It respects flowTypes, like get_subscription_usage.
• Failure handling: when its ClickHouse read fails, CustomersDot returns nonBillableUsage: null with the error nonBillableUsage is temporarily unavailable. The client logs a warning and returns success: false, without raising or reporting to Sentry, so the field resolves to null and the rest of subscriptionUsage still resolves. Any other error raises as before.
• Model: GitlabSubscriptions::SubscriptionUsage#non_billable_usage maps the response to NonBillableUsage Structs, reusing DailyUsage.
• GraphQL: new GitlabSubscriptionUsageNonBillableUsage type with:
  • productId: matches GitlabSubscriptionUsageProduct.id.
  • skipReason: nullable; null means the event has no recorded reason.
  • totalCreditsUsed
  • dailyUsage

These totals are not included in creditsUsed or dailyUsage, so billed totals still reconcile with Customers Portal.

The frontend (stacked chart in wallet_agnostic_credits_dashboard) follows in a separate MR, for multi-version compatibility.

References

• Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/632384
• Epic: https://gitlab.com/groups/gitlab-org/-/work_items/23876
• CustomersDot counterpart MR: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17365
• Billing engine writer of skip_reason (draft): https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17371
• Similar MR: !254129 (merged)

Screenshots or screen recordings

How to set up and validate locally

1. Run GDK in SaaS mode (export GITLAB_SIMULATE_SAAS=1 gdk restart).

2. Run CustomersDot locally on the branch from https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17365, connected to your GDK.

3. Have a group with GitLab Credits and generate usage, including some events with SkipConsumption = true in usage_billing_enriched (with and without a skip_reason key in Metadata).

4. As a group owner, run this in https://gdk.test:3443/-/graphql-explorer:

   query {
     subscriptionUsage(namespacePath: "test-group-hvs-101", startDate: "2026-10-01", endDate: "2026-10-06") {
       creditsUsed
       nonBillableUsage {
         productId
         skipReason
         totalCreditsUsed
         dailyUsage {
           date
           creditsUsed
         }
       }
     }
   }

5. Check that nonBillableUsage lists one entry per product and skip reason, skipReason is null for events without a reason, and creditsUsed is unchanged.

MR acceptance checklist

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