[jayzhan211]

Which issue does this PR close?

• None. This is a follow-up to test: add planner tests for ordering, constant and equality facts #26570, whose cross-join tests this PR relies on. This PR is stacked on test: add planner tests for ordering, constant and equality facts #26570, so
  only the last commit belongs to it.

Rationale for this change

CrossJoinExec computes its output facts (constants, equalities, orderings) by calling
join_equivalence_properties with JoinType::Full. A cross join never pads rows with NULLs, so a full outer join
is the wrong model. Today this does not change any plan, because the current outer-join rules keep the constants
that a cross join needs. But it ties cross join to rules that are about NULL padding, and those rules are being
changed: with #26459 or #26042 merged, a query such as

SELECT l.c, s.x
FROM (SELECT c FROM t) l CROSS JOIN (SELECT x FROM small WHERE x = 1) s
ORDER BY l.c, s.x;

sorts on x again even though x = 1 still holds after the join. The cross-join cases added in #26570 fail with
either PR merged in.

What changes are included in this PR?

CrossJoinExec now derives its equivalence properties as an inner join with no join keys, with a comment that
says why. Nothing else changes: with no keys and no maintained input order, an inner join and a full outer join
give the same facts under the current rules.

What is the testing strategy for this PR?

The cross-join cases in equivalence_properties.slt from #26570 cover this change. All sqllogictest files and the
join and physical optimizer tests pass unchanged. I also merged #26459 and #26042 into this branch one at a time:
without this change the cross-join cases fail with an extra sort, and with it they pass.

Are there any user-facing changes?

No.