PiecewiseMergeJoin reports a false equi-key equivalence for INNER range joins, dropping a required sort

[viirya]

Describe the bug

An INNER PiecewiseMergeJoin reports a false output equivalence between the two sides of its range predicate, which can make the optimizer drop a required sort and return wrongly ordered results.

PiecewiseMergeJoinExec::compute_properties passes the join's on pair to join_equivalence_properties as if it were an equijoin key. For an INNER join, join_equivalence_properties registers left_on == right_on as an output equivalence. But PWMJ's on is a range predicate (l.v < r.v), not equality — so this equivalence is false. A downstream ORDER BY on the "other" column can then be optimized away because the planner believes it is already sorted.

To Reproduce

set datafusion.optimizer.enable_piecewise_merge_join = true;

create table l(v int) as values (1),(2),(3),(5),(8);
create table r(v int) as values (4),(6),(9),(2);

select l.v as lv, r.v as rv
from l join r on l.v < r.v
where l.v = 2
order by r.v;

Result with PiecewiseMergeJoin:

+----+----+
| lv | rv |
+----+----+
| 2  | 9  |
| 2  | 6  |
| 2  | 4  |
+----+----+

The correct result (what NestedLoopJoin returns with the flag off) is ordered by rv:

+----+----+
| lv | rv |
+----+----+
| 2  | 4  |
| 2  | 6  |
| 2  | 9  |
+----+----+

The plans show why — PWMJ sorts only on l.v, dropping the r.v sort:

PWMJ:  SortExec: expr=[v@0 ASC]                 <- only lv, rv sort elided
NLJ:   SortExec: expr=[v@0 ASC, v@1 ASC]        <- both

Expected behavior

A range join adds no column equivalences. PiecewiseMergeJoin should match NestedLoopJoin; the ORDER BY r.v sort must be preserved.

Additional context

• Only INNER is affected — join_equivalence_properties/EquivalenceGroup::join only registers on-pair equalities for Inner.
• PiecewiseMergeJoin is experimental (enable_piecewise_merge_join defaults to false), but this is worth fixing before default-enablement.
• Part of the classic-join hardening for [EPIC]: Make PiecewiseMergeJoin work in Datafusion #17427. Fix + regression test coming.