CalendarEvent/get does not expose CalDAV x-href

### Description `CalendarEvent/get` does not return the corresponding CalDAV URL (`x-href`) for the event, making it difficult to correlate JMAP events with their CalDAV representations. Verified on v0.15.5. ### Use case When building a hybrid JMAP + CalDAV client (JMAP for writes, CalDAV for reads such as `<C:expand>` occurrence expansion), it is necessary to know the CalDAV URL of a JMAP-created event to issue targeted CalDAV requests. For example, Cyrus IMAP includes an `x-href` property in `CalendarEvent/get` responses pointing to the canonical `.ics` URL of the event (e.g. `/dav/cal/{user}/{calId}/{uid}.ics`). ### Suggestion Include `x-href` in `CalendarEvent/get` responses containing the canonical CalDAV URL for the event. ### Code of Conduct - [x] I agree to follow this project's Code of Conduct