Grandparent column reference in nested subquery causes planner error

### Describe the bug

```sql
CREATE TABLE t0 (g INT);
CREATE TABLE t1 (a INT);
CREATE TABLE t2 (c INT);
INSERT INTO t0 VALUES (1), (2);
INSERT INTO t1 VALUES (10);
INSERT INTO t2 VALUES (1);

-- Outer-outer column `t0.g` is referenced only at the deepest level
-- (t2.c = t0.g). The middle EXISTS (over t1) has no direct reference
-- to t0, so its Subquery.outer_ref_columns gets populated as empty.
SELECT g FROM t0 WHERE EXISTS (
SELECT 1 FROM t1 WHERE EXISTS (
SELECT 1 FROM t2 WHERE t2.c = t0.g
)
);
```

-->

```
Schema error: No field named t0.g. Valid fields are __correlated_sq_1.c.
```

### To Reproduce

_No response_

### Expected behavior

_No response_

### Additional context

_No response_