[persist] Fix a bug in the projection optimization refactoring

[bkirwi]

Two related bugs:

• The new optimization path was creating a column for K=Row, not K=SourceData. This would never work, so the optimization wasn't kicking in.
• The "faked" data didn't have a schema id, which caused issues at part decoding.

Motivation

https://github.com/MaterializeInc/database-issues/issues/9174

Tips for reviewer

There's some additional dedup and cleanup to be done here - I'll take that in a followup PR.

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[bkirwi]

Nightlies: https://buildkite.com/materialize/nightly/builds?branch=bkirwi%3Afix-migration

[ParkMyCar]

Nice!! Maybe also worth a backport to reinforce #32173?