`generate_subscripts()` returns subscripts that do not match `int2vector` subscripting

int2vector is 0-indexed, rather than 1-indexed like other array types in postgres. `generate_subscripts` needs to account for this.

Repro:

```sql
CREATE TABLE bug2 (a integer, b integer);
CREATE INDEX bug2_ab ON bug2 (a, b);

SELECT indkey, -- '1 2'
indkey[0], -- NULL <- subscripting is 1-based
indkey[1], -- 1
array(SELECT s FROM generate_subscripts(i.indkey, 1) AS g(s)) -- {0,1} <- 0-based
FROM pg_index i WHERE i.indexrelid = 'bug2_ab'::regclass;
```

Start by running the SQL reproduction and then locate the generate_subscripts implementation and existing tests for array subscripting. Update the behavior for int2vector so its generated subscripts match its indexing semantics, and add a regression test showing the corrected result.

Written by the indexing model from the issue text.