Expose non-billable usage on the personal credits dashboard API

What does this MR do and why?

The personal GitLab Credits dashboard (User settings > GitLab Credits, GitLab.com only) reads billable usage only, so a user's Orbit promotional usage is invisible there, while the group dashboard's Total usage card already counts it. CustomersDot exposes a per-user nonBillableUsage field since https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17539 (deployed). This MR reads it and exposes it on currentUser.creditsUsage.

• SubscriptionUsageClient#get_non_billable_usage_for_user_id: a new request for the user's nonBillableUsage, with the same NON_BILLABLE_USAGE_UNAVAILABLE handling and ClickHouse read timeout as the subscription-level get_non_billable_usage. It is a separate request from the daily usage because CustomersDot answers a ClickHouse failure on that field with an error, which would otherwise fail the billable series too.
• CreditsUsage#non_billable_usage: builds the per-product, per-skip-reason entries scoped to the user. Returns nil (not []) when CustomersDot can't read the data, so the API can tell "none" from "unknown".
• GitlabSubscriptionUserCreditsUsageNonBillableUsage: a new GraphQL type that authorizes read_user, like the other self-view types, with productId, skipReason, totalCreditsUsed and dailyUsage.
• nonBillableUsage field on GitlabSubscriptionUserCreditsUsage, nullable.
• Specs and regenerated GraphQL docs.

This is change B of the plan in #633154 (comment 4099136079). The frontend (C) follows in a separate MR on top of this branch. No existing field changes meaning: creditsUsed and dailyUsage stay billable-only.

References

• Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/634215
• Related to #633154 (closed)
• Plan: #633154 (comment 4099136079)
• Parent epic: https://gitlab.com/groups/gitlab-org/-/work_items/24080
• CustomersDot field: https://gitlab.com/gitlab-org/customers-gitlab-com/-/merge_requests/17539

Screenshots or screen recordings

How to set up and validate locally

1. Set up GDK in SaaS mode with a local CustomersDot, as in "How to set up and validate locally" in !260725 (merged).

2. In CustomersDot, seed billable and non-billable usage for one user of a subscription and run enrichment, for example:

   bin/populate_usage_data -r 20 --realm SaaS --root-namespace-id <id> --user-id <user_id> --feature-qualified-name orbit_mcp --team-member --skip-reason promo
   bin/populate_usage_data -r 20 --realm SaaS --root-namespace-id <id> --user-id <user_id> --feature-qualified-name orbit_rest

3. As that user, run this query in /-/graphql-explorer:

   query {
     currentUser {
       creditsUsage(namespacePath: "<group-path>") {
         creditsUsed
         dailyUsage { date creditsUsed }
         nonBillableUsage {
           productId
           skipReason
           totalCreditsUsed
           dailyUsage { date creditsUsed }
         }
       }
     }
   }

4. creditsUsed and dailyUsage are unchanged (billable only); nonBillableUsage lists the skipped usage per product and skip reason.

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