extra-user-roles=admin does not grant "CREATE USER" privileges

## Steps to reproduce

1. Following the tutorial, deploy a PostgreSQL <> Data-Integrator <> PGBouncer setup
2. Set `extra-user-roles` config on `data-integrator` to `admin`.
3. Get a PSQL shell using the credentials returned by `get-credentials` action on `data-integrator`.
4. Try running
```
CREATE ROLE myuser LOGIN PASSWORD 'strong_password';
````

which will error out with:

```
launchpad=> CREATE ROLE myuser LOGIN PASSWORD 'strong_password';
ERROR: permission denied to create role
DETAIL: Only roles with the CREATEROLE attribute may create roles.
```

## Expected behavior

User gets created successfuly and `admin` role has all the necessary privileges to GRANT necessary permissions for users.

1. CONNECT privilege
2. SCHEMA privileges
3. TABLE access related privileges

## Actual behavior

```
psql -U relation_id_9

launchpad=> CREATE database mydb;
CREATE DATABASE

launchpad=> CREATE ROLE myuser LOGIN PASSWORD 'strong_password';
ERROR: permission denied to create role
DETAIL: Only roles with the CREATEROLE attribute may create roles.

launchpad=> \du+
List of roles
Role name | Attributes | Member of | Description
----------------------------+------------------------------------------------------------+--------------------------------------+-------------
admin | Cannot login | {pg_read_all_data,pg_write_all_data} |
pgbouncer_auth_relation_10 | Superuser | {} |
relation_id_9 | Create DB | {admin} |

launchpad=>
```
## Versions

Operating system:
Ubuntu 24.04.2 LTS

Juju CLI: 3.6.5-genericlinux-amd64

Juju agent: 3.6.5

```
App Version Status Scale Charm Channel Rev Exposed Message
data-integrator active 1 data-integrator latest/edge 151 no
pgbouncer 1.21.0 active 1 pgbouncer 1/edge 744 no
postgresql 16.8 active 3 postgresql 16/edge 610 no
```

LXD: 5.21.3 LTS

## Log output

Juju debug log:

## Additional context

Start by reproducing the PostgreSQL and Data Integrator setup, then trace the implementation of the `extra-user-roles` configuration and the credentials returned by `get-credentials`. Done means the configured `admin` role permits CREATE ROLE and the listed CONNECT, schema, and table privileges, with coverage for the reported setup.

Written by the indexing model from the issue text.