S3 backups fail on the non-primary site if cross-site replication is enabled with the error message `Stanza was not initialised`

## Steps to reproduce

1. Set up cross-site replication in postgresql.
2. Set up s3-integrator charm.
3. `juju run -m pgsddev-swarm-a postgresql/0 create-backup` fails on all units in the non-primary postgresql model with a confusing message
```
$ juju run postgresql/0 create-backup
Running operation 71 with 1 task
- task 72 on unit-postgresql-0

Waiting for task 72...
Action id 72 failed: Stanza was not initialised

$ juju run postgresql/1 create-backup
Running operation 73 with 1 task
- task 74 on unit-postgresql-1

Waiting for task 74...
Action id 74 failed: Stanza was not initialised

```

Backups work as expected on the primary site.

## Expected behavior

Either the backups should work or the error message should be clearer if it's not supported.
Also, this results in a number of COS alerts about `PgBackRestBackupTooOld` since the backups don't work.

## Actual behavior
Explained above `Action id xx failed: Stanza was not initialised` on all non-primary-site units.

## Versions

Operating system: Ubuntu 24.04.3 LTS

Juju CLI: 3.5.7-genericlinux-amd64

Juju agent: 3.6.7

Charm revision: 16/stable 952

LXD: 5.21.4 LTS

## Log output

Juju debug log:
```
unit-postgresql-0: 15:25:25 INFO unit.postgresql/0.juju-log backups:A full backup has been requested on unit
unit-postgresql-0: 15:25:25 ERROR unit.postgresql/0.juju-log backups:Backup failed: Stanza was not initialised
```

## Additional context

Reproduce the failure by enabling cross-site replication and the s3-integrator charm, then run the create-backup action on both primary and non-primary PostgreSQL units. Start with the reported Juju debug-log lines and compare backup behavior between sites. Done means backups work on the non-primary site or the action reports a clear, supported limitation instead of `Stanza was not initialised`.

Written by the indexing model from the issue text.