Display error if user tries to create subsources for single-output load generators

[martykulma]

Return appropriate error if user tries to create a single-output load generator with subsources (e.g. FOR ALL TABLES, FOR TABLES ..., FOR SCHEMAS ...)

Motivation

Fixes https://github.com/MaterializeInc/database-issues/issues/9028

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.

[aljoscha]

This does what it says on the tin, and the code is good!

@petrosagg might want to look at this because he's currently in the business of moving all our sources to only allow the CREATE TABLE FROM SOURCE ... syntax. Which I'm thinking currently doesn't work with these load generator sources, maybe?

[aljoscha]

This one is longer because it first adds test coverage that wasn't there before, right?

[martykulma]

Correct, just keeping parity with testdrive, but if we don't need to, I'm happy to drop this.

@def-, should I be adding this testing here, or is it overkill?

[def-]

It doesn't hurt much either way, but no need to add a separate file here if there is nothing interesting being exercised. All of these old-kafka-src-syntax tests was supposed to be long removed, but then the new CREATE TABLE ... FROM SOURCE syntax work was paused for a while.

[petrosagg]

Disallowing this syntax in the current version of the product is inline with the migration so no concerns from me

[def-]

It doesn't hurt much either way, but no need to add a separate file here if there is nothing interesting being exercised. All of these old-kafka-src-syntax tests was supposed to be long removed, but then the new CREATE TABLE ... FROM SOURCE syntax work was paused for a while.

[def-]

could be more verbose with what the error message is