Enhance table scan unparsing to avoid unnamed subqueries.

[goldmedal]

Which issue does this PR close?

It's a follow-up PR of #12896

Rationale for this change

from https://github.com/apache/datafusion/pull/12896#issuecomment-2408609989

What changes are included in this PR?

Are these changes tested?

Enhance the original tests

Are there any user-facing changes?