[substrait] [sqllogictest] Cannot convert Exists { subquery: <subquery>, negated: true } to Substrait

[gabotechs]

Describe the bug

~30 sqllogictest cases are failing in Substrait round-trip mode because of the following error:

query failed: DataFusion error: This feature is not implemented: Cannot convert Exists { subquery: <subquery>, negated: true } to Substrait

To Reproduce

cargo test --test sqllogictests -- --substrait-round-trip joins.slt:1233

Expected behavior

The sqllogictest passes in Substrait round-trip mode

Additional context

This is part of a broader initiative to improve the reliability of Substrait-to-DataFusion logical plan conversions.

• [Epic] A collection of Substrait conversion issues #16248