Payment System (Friend Money Transfer)

Implement a simple in-memory payment system that supports user sign-up, friend management, and money transfers between friends. You will receive a list of request strings; each string is formatted as "requestId/action/parameters...". The actions are: CREATE_USER (add a new user with a starting balance), REQUEST_FRIEND (user A asks to be friends with user B), ACCEPT_FRIEND (user B accepts the pending request), and SEND_MONEY (user A transfers an amount to friend B). For every request you must return a string "requestId:result" where result is one of SUCCESS, FAILURE, or ERROR. A request fails (FAILURE) if any business rule is violated (duplicate user, non-existent user, insufficient balance, users not friends, amount ≤ 0, etc.). ERROR is reserved for malformed input. Process requests in the given order and emit the corresponding result for each one.