Add additional subquery sqllogictest coverage

Add subquery_additional.slt with ~59 test cases across 22 categories
to complement the existing subquery.slt. Tests are adapted from
DuckDB's scalar subquery test suite to improve result-correctness
coverage for subquery patterns not exercised by the plan-focused
tests in subquery.slt.

Categories covered:
- Basic and nested scalar subqueries (uncorrelated)
- Correlated scalar subqueries with aggregates
- Nested correlated subqueries (2-4 levels deep)
- EXISTS / NOT EXISTS (correlated and uncorrelated)
- IN / NOT IN with correlated filters
- Subqueries with DISTINCT, COALESCE, CASE WHEN
- Subqueries in HAVING, BETWEEN, expression contexts
- Scalar subqueries with CTEs
- Subqueries with ORDER BY / LIMIT
- UNION / UNION ALL inside subqueries
- NULL handling edge cases
- Subqueries with window functions
- Multiple scalar subqueries in SELECT
- Correlated subqueries with GROUP BY in outer query

No engine, planner, optimizer, or execution changes.
Test-only change.

Closes #20240

Author: Tushar7012