I wonder if escorts regularly take STI tests? Can I communicate with her about the earliest meeting time after she has completed her STI test? Would this be offensive? (I can also provide my report.)

I usually see WG that are very open about that matter as they do regular check-ups. I think it's a normal thing to ask in this business and many of them are responsible about their health. 
IMO, if she is offended because of it, then you should avoid that kind of users as she is probably hidding something or not taking tests. 

If you can't talk freely it wil be a bad meet, they are used to this type of question and a lot more