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

[przemeklal]

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

[syncronize-issues-to-jira]

Thank you for reporting your feedback to us!

The internal ticket has been created: https://warthogs.atlassian.net/browse/DPE-9010.

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