Creates a new chat and returns it in full, exactly as a subsequent GET of the chat would.
members states who is in the chat from creation: each named principal is joined outright, on an explicit grant recorded alongside. Nobody else is put in the chat, the creator included, so name the calling app here if it is to take part. At least one member is required.
Private chats created without a title are unique per set of members: creating another untitled chat with the same members returns the existing chat rather than creating a duplicate. When a title is supplied a new chat is always created. The creator, attributed on the chat's automatically generated header message, is always the caller's authenticated identity and never anything in the request body.