Support vector index build under the legacy schema changer

Issue #149236 reported a nilptr exception while creating a vector index. Further investigation reveals that vector index backfill is unimplemented under the legacy schema changer. While this is a feature we would like to add, it doesn't make sense to rush it in for 25.3 since most backfill is done with the declarative schema changer.

When implemented, the following snippet should properly create a vector index:
```
set autocommit_before_ddl=off;
begin;
create table exec_test (a int primary key, b int, vec1 VECTOR(3));
create VECTOR INDEX idx1 ON exec_test (vec1) WITH (min_partition_size=1, max_partition_size=4);
commit;
```

Hat tip to @spilchen for providing a simple test case.

Jira issue: CRDB-52596