Session-start sync for native in-app messages. The mobile SDK calls this once per session; the request is forwarded to the messages microservice, which evaluates eligibility, assigns A/B variations (including control holdouts) and returns pre-rendered messages the SDK caches and displays offline. Follows the V1 bots identity convention: the player arrives as a plain external id plus an optionally provided encrypted internal id, mirroring the balance and levels endpoints, with the encrypted id taking precedence when present.