I'm about to start a game this weekend, and it was unclear to me if there's interaction with the Naalu's "Foresight" racial ability and PDS fire.

When they use the CC from their strategy pool and put it in an adjacent system, does that "activate" the system for the purpose of PDS II (space cannon) fire? Or are they safe from that type of fire.

No, it only locks the system they retreated to so they can't move those ships again unless they have the "Unexpected Action" card which let's them pick up a token from the board, or they have and use the primary ability of Warfare to pick up a token from the board. PDS shots typically only fire into activated systems.

To further expand on the PDS defensive or offensive shots: PDS fire occurs during the Movement step of the Action phase. This step occurs at the beginning of the active player's activation of the system.