Charm relation hooks fail permanently if a user executes DROP SCHEMA public CASCADE

## Steps to reproduce

1. Deploy the `postgresql` charm (e.g., channel `16/stable`) and relate it to an application.
2. Connect to the resulting application database using the credentials created by the relation.
3. Execute `DROP SCHEMA public CASCADE;`
4. Remove the Juju relation with the application.
5. Re-integrate the application with the database (or relate a new application).
6. Observe that the relation hangs indefinitely, with the application waiting for a database connection string.

## Expected behavior
If possible, the charm's internal mechanics should be resilient to standard database cleanup commands run by application users. A user dropping their application schemas (like the default `public` schema) should not delete the charm's internal role-management functions or break its ability to manage Juju relations.

## Actual behavior

The internal function `set_up_predefined_catalog_roles` is deleted because it resides in the `public` schema. When the charm attempts to establish the new relation, it fails to execute the required role-setup function and crashes the hook. The application is left hanging indefinitely waiting for connection credentials.

## Possible fixes

Would either of these be possible to implement?
- Move `set_up_predefined_catalog_roles` and other charm-critical functions out of the `public` schema into a dedicated, protected schema (e.g., `charmed_internal`) or
- Update relation hooks to verify the existence of `set_up_predefined_catalog_roles` and automatically copy it back from `template1` if missing.

## Versions

Operating system:

Juju CLI: 3.6.23-genericlinux-amd64

Juju agent: 3.6.12

Charm revision: 16/stable rev 1047

## Log output

Juju debug log:

```
unit-postgresql-5: 02:51:24 ERROR unit.postgresql/5.juju-log database-peers:35: single_kernel_postgresql.utils.postgresql:Failed to create database: function set_up_predefined_catalog_roles() does not exist
LINE 1: SELECT set_up_predefined_catalog_roles();
...
ValueError: Cannot change extra-user-roles after relation has already been created

```
more extensive logs https://pastebin.canonical.com/p/rRbRBZnpNM/

## Additional context