[CT-2325] Validate public models are not materialized as ephemeral

[MichelleArk]

Ephemeral materializations shouldn't be made public because there is no database identifier that gets materialized in the data warehouse for use as a 'production pointer' by downstream dbt projects.

This could be a parse-time error (similar to what's been proposed for #7063 (comment)), or optionally could be a warning surfaced prior to writing a publication artifact that excludes any public models with ephemeral materializations. Either way, public models with ephemeral materializations should not be included as part of the publication artifact because there will be no way to resolve the reference from a downstream project.