adbc_driver_postgres flatten multi dimensional array in Postgres

### What happened?

When querying a multi dimensional array in Postgres the returned result "loose" the dimensions, e. g. a 2 dimension array is returned as 1 dimension array, the query:

```SQL
select array[[1, 1], [2, 3]]
```

It should return:
```python
[([[1, 1],[2, 3]],)]
```

It returns:
```python
[([1, 1, 2, 3],)]
```

### Stack Trace

_No response_

### How can we reproduce the bug?

```python
import adbc_driver_postgresql.dbapi

conn_string = 'postgresql://postgres:XXXX@localhost:5432/postgres'
conn = adbc_driver_postgresql.dbapi.connect(conn_string)

cursor = conn.cursor()
query = 'select array[[1, 1], [2, 3]]'
cursor.execute(query)
cursor.fetchall()
```

### Environment/Setup

macOS
PostgreSQL 15
adbc-driver-postgresql 1.1.0

Reproduce the issue through adbc_driver_postgresql.dbapi.connect using the multidimensional PostgreSQL array query shown. Trace the PostgreSQL DBAPI result conversion and ensure the fetched value preserves nested array dimensions, then verify it matches the expected Python result.

Written by the indexing model from the issue text.