planner: add right join regression coverage for constant expression index outer join

## Enhancement

Track a planner follow-up to add symmetric `RIGHT JOIN` regression coverage for the constant-expression index outer-join case related to `#66859`.

### Background

- The current regression coverage already protects the `LEFT JOIN` shape for the constant-expression index scenario.
- The symmetric `RIGHT JOIN` shape exercises the same outer-join directionality and should stay covered explicitly as a separate follow-up.
- This issue is for coverage parity only; it is not intended to reopen the already-merged fix path.

### Suggested scope

- Add a `RIGHT JOIN` regression next to the existing `LEFT JOIN` assertion in `pkg/planner/core/casetest/join/join_test.go`.
- Keep the change test-only unless the new regression exposes a distinct planner hole.

### Reference SQL

```sql
drop table if exists issue66859_t0, issue66859_t1, issue66859_t2;
create table issue66859_t0(c0 int);
create table issue66859_t1 like issue66859_t0;
create index i0 on issue66859_t1((5));
insert into issue66859_t0(c0) values(-1);

select issue66859_t0.c0 as ref0, issue66859_t1.c0 as ref2
from issue66859_t1 right join issue66859_t0 on issue66859_t0.c0 = issue66859_t1.c0
where 5 >= issue66859_t0.c0;
```

### Expected result

- `-1 `

### Related context

- `#66859`