### Release notes Add a REST API endpoint equivalent to the `processUserBillablePromotionRequest` GraphQL mutation, enabling clients to process user billable promotion requests without using GraphQL. ### Problem to solve Currently, the only way to programmatically process user billable promotion requests is through the `processUserBillablePromotionRequest` GraphQL mutation. Clients who primarily use the REST API are forced to integrate GraphQL solely for this operation, which adds unnecessary complexity and friction — especially if this is the only GraphQL call they would need to make. ### Proposal Introduce a new REST API endpoint that provides the same functionality as the `processUserBillablePromotionRequest` GraphQL mutation. This endpoint should accept the same parameters and return equivalent response data, allowing clients to process user billable promotion requests entirely through the REST API. ### Intended users - API integrators and developers who build tooling on top of GitLab's REST API - Administrators managing user billable promotions programmatically - Clients and third-party applications that exclusively use the REST API ### Feature Usage Metrics Usage of the new REST API endpoint can be tracked alongside existing metrics for the `processUserBillablePromotionRequest` GraphQL mutation to compare adoption across both interfaces. ### Does this feature require an audit event? This should align with any existing audit events already associated with the `processUserBillablePromotionRequest` GraphQL mutation. If the mutation currently triggers an audit event, the REST endpoint should do the same.