Add locationId to Credits usage event GraphQL type

What does this MR do and why?

GitlabSubscriptionUsageUserEvent#location resolves a Credits usage event's project or group through a batch loader. When that record has been deleted — or the viewer cannot see it — the field resolves to null, and the raw reference the historical billing event still carries becomes invisible to API consumers. A valid credit charge then looks unattributable.

This MR adds locationId, a Global ID returned regardless of whether the record resolves. It follows the same project-over-namespace precedence as #location, so the two fields always describe the same record.

Why clients can safely branch on this

location is a Project | Group union, and both members carry type-level authorization (read_project / read_group). When the viewer lacks access, GraphQL nulls the field out before any resolver code could observe why — so the backend cannot cheaply distinguish "deleted" from "not visible".

That is deliberate here. Clients can read location == null && locationId != null as a single state covering both causes, without the backend duplicating a permission check or disclosing which of the two applies. This matches how GitLab answers 404 rather than 403 for records the viewer may not be allowed to know about.

Split from the frontend change

Per the multiversion compatibility guidance, this backend-only MR must be merged and deployed before the frontend consumes the field. @gl_introduced covers GitLab Self-Managed but explicitly does not mitigate the GitLab.com canary window.

The frontend follow-up is !249076 (closed).

References

#608133

Screenshots or screen recordings

No user-visible change in this MR — it only adds a GraphQL field. The UI change and its screenshots are in the frontend follow-up.

How to set up and validate locally

query {
  subscriptionUsage(namespacePath: "<group>") {
    usersUsage {
      users(username: "<username>") {
        nodes {
          events {
            nodes {
              location { __typename ... on Project { id name } ... on Group { id name } }
              locationId
            }
          }
        }
      }
    }
  }
}

locationId is populated for every event that carries a project_id or namespace_id, including events whose location comes back null.

MR acceptance checklist

• Unit tests cover the three #location_id branches: project ID present, only a namespace ID, and neither.
• Request specs cover a resolvable project, a deleted project, an inaccessible private project, and a deleted group.
• doc/api/graphql/reference/_index.md and public/-/graphql/introspection_result.json regenerated for the new field.
• No feature flag: the field is additive and cannot change an existing response.
• No database or migration changes.