Wrong results: eliminate_cross_join drops an equi-join key that spans two cross-joined relations #26058

Describe the bug

An inner join whose ON has an ordinary equi-key plus a second equality whose one side spans two relations of a cross join loses the second equality. The query returns rows that do not satisfy the join condition.

extract_equijoin_predicate turns b.id = CASE k.k WHEN 0 THEN a.x ELSE a.y END into an equi-join key whose left expression references both a and k. eliminate_cross_join then flattens (a CROSS JOIN k) JOIN b into one join graph and rebuilds it as (a JOIN b ON a.t = b.t) CROSS JOIN k. The key that references both a and k is not attached to any join and is not kept as a filter.

Reproduced on 55.1.0 and on main (8248a57). Running the query with each logical optimizer rule removed in turn, it is correct without extract_equijoin_predicate or without eliminate_cross_join, and wrong without any other single rule.

To Reproduce

SELECT k.k, b.id
FROM (SELECT 1 AS t, 17 AS x, 18 AS y) a
CROSS JOIN (SELECT 0 AS k UNION ALL SELECT 1) k
JOIN (SELECT 1 AS t, CAST(value AS INT) AS id FROM generate_series(0, 20)) b
  ON b.t = a.t AND b.id = CASE k.k WHEN 0 THEN a.x ELSE a.y END
ORDER BY k.k, b.id;

returns 42 rows: every b.id from 0 to 20, once for each k.

The optimized logical plan has no trace of the CASE predicate:

Sort: k.k ASC NULLS LAST, b.id ASC NULLS LAST
  Projection: k.k, b.id
    Cross Join:
      Projection: b.id
        Inner Join: a.t = b.t
          SubqueryAlias: a
            Projection: Int64(1) AS t
              EmptyRelation: rows=1
          SubqueryAlias: b
            Projection: Int64(1) AS t, CAST(generate_series().value AS Int32) AS id
              TableScan: generate_series() projection=[value]
      SubqueryAlias: k
        Union
          ...

Expected behavior

Two rows, (0, 17) and (1, 18).

Without b.t = a.t in the ON clause the result is correct, as it is when the CASE predicate is written in a WHERE instead of the ON clause.

Additional context

Found through Sail, where a query joining a table on s.tic = rt.tic AND s.id = CASE k.k WHEN 0 THEN rt.sector_id ELSE COALESCE(rt.spawn_sector_id, rt.sector_id) END over a cross join with k returned every row for that tic.