By Juliana Liu   China has never been shy about its commercial aviation ambitions. For decades, it has wanted to be the “C” in the ABCs of aerospace after Airbus SE and Boeing Co. But to achieve that goal, it must solve an engineering puzzle that only a handful of companies have ever cracked: building a reliable jet engine.    The risks of not doing so are evident — 2025 was supposed to have been the year that Commercial Aircraft Corp of China Ltd., a state-owned aerospace manufacturer leading the self-s ...