create business event service cannot express unlimited attributes, collides with the CloudEvent id attribute, leaves V2 subject and channel name unmanaged #1525

Summary

create business event service cannot express unlimited attributes, collides with the CloudEvent id attribute, and leaves the V2 subject and the channel name to a manual patch. Related, closed: #476 (reserved message attribute names), PR #533 (duplicate channels crash in the catalog).

Repro

1. An app with the Business Events module. create business event service M.Svc ... message Msg ( EventId: String, Data: String );
2. A long payload needs an unlimited attribute; MDL only offers String(200).
3. Publishing with PublishBusinessEvent_V2: a Subject written via MDL needs a refresh (CE0115).
4. Create the same service in a second app: the ChannelName differs per service.

Expected

A buildable publisher and subscriber from MDL alone: an unlimited length, a check diagnostic for the reserved CloudEvent member name, the V2 subject written consistently, and a way to state the ChannelName.

Actual

String(200) only; EventId duplicates an inherited CloudEvent member (reported by mxbuild); the V2 subject needs repair; the channel name is random, so two apps land on different topics.

Tested with: Mendix 11.15.0, mxcli v0.26.0 on macOS (code also read in the v0.27.0 source). Not retested on v0.27.0.