Set User Synced Attributes

Sets attributes on the user via the events pipeline — the same path SDK user_attributes events take — so values land in both the subscriptions store and analytics. Propagation is asynchronous (typically seconds). The accepted field in the response means the request passed validation and was handed to the pipeline — it is not a read-back of the stored snapshot; use GET /v3/users/{app_user_id}/attributes to observe persisted values. Requires users:write scope.