>could file a civil suit against OpenAI,

Everyone can sue everyone, there's no prohibition on suing someone, what changes is whether the case is good (has a reasonable chance of favourable sentence)

That said, it is often unclear whether an agent is operated by the model manufacturer (for example by scraping a website), or acting on behalf of a user.

In the former ofc the proper defendant is OAI. On the second, the argument for suing OAI is weak, the most natural defendant is the user that prompted the agent. If the facts later reveal that there was no malicious intent, then you can retarget the defendant.