fix: correlated exists/not exists subqueries hit the count bug for groupless aggregates

[mohammadnaqvi04]

Which issue does this PR close?

• Part of Correlated EXISTS/IN subqueries with groupless aggregates hit the count bug #24960.

Rationale for this change

See #24960.

This is a split of #25008, which originally covered EXISTS/NOT EXISTS/IN/NOT IN together. That PR has been split into two smaller PRs between this one for EXISTS/NOT EXISTS and a (to come) follow-up stacked on top of it for IN/NOT IN.

What changes are included in this PR?

Correlated EXISTS/NOT EXISTS subqueries had count-bug compensation available but never turned it on.

• Turn on count-bug compensation for correlated EXISTS/NOT EXISTS subqueries.
• When compensation is needed, decorrelate into a LEFT JOIN and substitute the correct value instead of dropping the row.
• Make unsupported join shapes fail loudly instead of silently falling back to incorrect behavior.

IN/NOT IN compensation is out of scope for this PR and returns an explicit "not implemented" error. This will be covered by the follow-up PR.

What is the testing strategy for this PR?

Added twenty sqllogictest cases in subquery.slt.

Ran all datafusion, datafusion-cli, and datafusion-sqllogictest suites, and ./dev/rust_lint.sh.

Also differentially fuzz-tested against DuckDB with a standalone harness, and did several adversarial review passes with Claude to look for gaps beyond the fuzzer's generated shapes.

Are there any user-facing changes?

None, except in the sense that this fixes a user-visible bug.

AI Usage

AI was used during development of this PR, primarily to help review the test suite and verify test cases against the implemented fix.

[mohammadnaqvi04]

This decorrelates a groupless-aggregate subquery to a LEFT JOIN, grouped by the correlated column, with the unmatched-row NULL replaced by the aggregate's empty-input default (e.g. count(*) → 0, sum(x) -> NULL).
For

SELECT t1.t1_int FROM t1 WHERE EXISTS (
    SELECT count(*) FROM t2 WHERE t1.t1_int = t2.t2_int
);

this is equivalent to

SELECT t1.t1_int
FROM t1
LEFT JOIN (SELECT t2_int, count(*) AS cnt FROM t2 GROUP BY t2_int) AS sq
  ON t1.t1_int = sq.t2_int;

[mohammadnaqvi04]

LIMIT 0 on the subquery collapses it to an EmptyRelation before the count-bug join ever sees it, so without this flag, join-compensation has no way of knowing that the subquery is unconditionally empty and defaults every row to matched. For

SELECT t1.t1_int FROM t1 WHERE EXISTS (
    SELECT count(*) FROM t2 WHERE t1.t1_int = t2.t2_int LIMIT 0
);

t1_int should never appear in the result, since LIMIT 0 empties the subquery regardless of whether t2 has a matching row. Setting forces_empty_result is what makes build_join_with_count_bug return false unconditionally instead of falling back to its usual "matched" default.