sql: unsafe enum use in transaction

The following behavior is not allowed in postgres:

```
CREATE TYPE bogus AS ENUM('good');
BEGIN;
ALTER TYPE bogus ADD VALUE 'new';
SAVEPOINT x;
SELECT enum_last(null::bogus); -- unsafe use of new value "new" of enum type bogus
ROLLBACK TO x;
SELECT enum_range(null::bogus);-- unsafe use of new value "new" of enum type bogus
ROLLBACK TO x;
COMMIT;
-- Now the commands can succeed.
```

In crdb, we return `good` and `{good}`, respectively, instead of an error.

Found in pg_regress enum

Jira issue: CRDB-33908

Epic CRDB-60818