How to accept or decline ICS file invitations in Calendar on Mac?

I have received a mail message with an event invitation as an .ICS attachment. I followed the instructions in Use Siri Suggestions in Calendar on Mac. The invitation does not appear in Calendar (iCalendar). I can double-click on the .ICS file but then I get a read-only event in Calendar and no option to accept or decline.

The above support article has "Event details from known providers that you receive in other apps, such as Mail, automatically appear in Calendar as suggested events" (my emphasis). What may be happening is that my "provider" is not known. If that is the mail address, then almost all will be unknown, so it is more likely something inside the .ICS file, maybe this: "PRODID:-//The Horde Project//Horde iCalendar Library//EN".

I could test this if anyone could give me a PRODID known to work.

My Calendar synchronises via iCloud with an iPhone. How do I make my Mac running Sequoia 15.3.2 able to accept and decline invitations in mail?