BOS-IST flights are DL flights operated by DL
IST=CDG is an AF flight, operated by AF
CDG-BOS is operated by AF, but can either be booked as an AF flight or a DL codeshare. Price is the same.
All segments are in Business class, but on the DL website and also speaking with an agent, the return fares are in a higher class than the outgoing fares. Neither I nor the agent can pull up the less expensive fare class.