Fix Unparser for stacked Projections & Expand Unparser tests

[cetra3]

Which issue does this PR close?

Fixes #21066

Rationale for this change

The Unparser breaks in some certain scenarios, which also don't have test coverage.

What changes are included in this PR?

• Fix: dequalify outer SELECT columns when wrapping stacked projections as derived subqueries
• Tests: parameterize unparser roundtrip tests over all dialects + optimized/unoptimized plans, add regression test suite

Are these changes tested?

Yes, they are tested, the majority of this PR is to introduce more test paths.

Are there any user-facing changes?

No breaking changes, but some SQL generated by the Unparser may end up being a little different

[cetra3]

OK adding the optimized pass and different dialects has actually opened a can of worms for unparser corner & edge cases. I will see if I can work through these in this PR and if not, maybe will need to open a more conservative targeted PR