GitLab 18.11: Budget guardrails for GitLab Credits

Introduced subscription-level spending caps for GitLab Credits, allowing billing account managers to set a monthly ceiling for on-demand credit consumption. Implemented per-user credit limits, configurable via the GitLab GraphQL API, to prevent individual users from disproportionately consuming credits. Added visibility and notifications for cap status and enforcement, including email alerts to billing account managers when subscription-level caps are reached.