When they say "file for bankruptcy", the really mean "seek creditor protection". Under what is commonly referred to as "Chapter 11" in the USA and "CCAA" in Canada (Companies and Creditors Arrangement Act).

The purpose is to give a company the opportunity to restructure its debt and operations as an ongoing concern. In other words, nothing changes with respect to customers in the immediate term. Although, there could be flight cutbacks and rescheduling in the short-term.

It is when restructuring as an ongoing concern doesn't work that you need to worry, since pre-paid tickets would be just another debt of an unsecured creditor.