Cannot have backups when using s3-integrator 2/edge

## Steps to reproduce

```bash
juju deploy postgresql --channel 16/stable --constraints="cores=4 mem=4G"
juju deploy s3-integrator --base=ubuntu@24.04 --channel 2/edge --revision 167
secret=$(juju add-secret minio-postgres access-key=admin secret-key=password)
juju grant-secret minio-postgres s3-integrator
juju config s3-integrator \
endpoint="https://10.240.246.4:9000" \
bucket=foobar \
path=/postgresql \
s3-uri-style="path" \
s3-api-version="" \
region="" \
tls-ca-chain="$(base64 -w0 cert.pem)" \
credentials=$secret
juju integrate s3-integrator postgresql
```

## Expected behavior

Postgres detects the bucket that is pre-created in Minio, and then it writes the required files to setup a backup location.

## Actual behavior

The backup init is starting successfully but a warning message is thrown in the mid of the process. It seems that the postgres is setting a bucket name in the relation databag which is the relation id: `relation-17`. Then the s3-integrator is consuming it, and it is changing the bucket location. This bucket does not exist in the s3 location, so backup init is stuck.

## Versions

Operating system: Ubuntu 24.04

Juju CLI: 3.6.8-genericlinux-amd64

Juju agent: 3.6.8

Charm revision: 843

LXD: 5.21.3 LTS

## Log output

Juju debug log:

```
unit-s3-integrator-1: 08:28:23 INFO juju.worker.uniter.operation ran "update-status" hook (via hook dispatching script: dispatch)
unit-s3-integrator-1: 08:29:14 INFO juju.worker.uniter.operation ran "config-changed" hook (via hook dispatching script: dispatch)
unit-s3-integrator-1: 08:30:18 INFO juju.worker.uniter.operation ran "s3-credentials-relation-created" hook (via hook dispatching script: dispatch)
unit-s3-integrator-1: 08:30:19 INFO juju.worker.uniter.operation ran "s3-credentials-relation-joined" hook (via hook dispatching script: dispatch)
unit-s3-integrator-1: 08:30:19 INFO unit.s3-integrator/1.juju-log s3-credentials:17: On storage-connection-info-requested
unit-postgresql-1: 08:30:20 INFO juju.worker.uniter.operation ran "s3-parameters-relation-created" hook (via hook dispatching script: dispatch)
unit-s3-integrator-1: 08:30:20 INFO juju.worker.uniter.operation ran "s3-credentials-relation-changed" hook (via hook dispatching script: dispatch)
unit-postgresql-1: 08:30:39 INFO unit.postgresql/1.juju-log s3-parameters:17: backups:Bucket foobar exists.
unit-postgresql-1: 08:30:40 INFO unit.postgresql/1.juju-log s3-parameters:17: backups:Reading content from bucket=foobar, path=postgresql/model-uuid.txt
unit-postgresql-1: 08:30:40 INFO unit.postgresql/1.juju-log s3-parameters:17: backups:No such object to read from S3 bucket=foobar, path=postgresql/model-uuid.txt
unit-postgresql-1: 08:31:00 INFO unit.postgresql/1.juju-log s3-parameters:17: backups:Uploading content to bucket=foobar, path=postgresql/model-uuid.txt
unit-postgresql-1: 08:31:01 INFO juju.worker.uniter.operation ran "s3-parameters-relation-changed" hook (via hook dispatching script: dispatch)
unit-postgresql-1: 08:31:02 INFO unit.postgresql/1.juju-log s3-parameters:17: root:Starting rotate logs process
unit-postgresql-1: 08:31:02 INFO unit.postgresql/1.juju-log s3-parameters:17: root:Started rotate logs process with PID 26749
unit-postgresql-1: 08:31:03 INFO juju.worker.uniter.operation ran "s3-parameters-relation-joined" hook (via hook dispatching script: dispatch)
unit-s3-integrator-1: 08:31:03 INFO unit.s3-integrator/1.juju-log s3-credentials:17: On storage-connection-info-requested
unit-s3-integrator-1: 08:31:04 INFO juju.worker.uniter.operation ran "s3-credentials-relation-changed" hook (via hook dispatching script: dispatch)
unit-postgresql-1: 08:31:23 WARNING unit.postgresql/1.juju-log s3-parameters:17: backups:Bucket relation-17 doesn't exist or you don't have access to it.
unit-postgresql-1: 08:31:24 INFO juju.worker.uniter.operation ran "s3-parameters-relation-changed" hook (via hook dispatching script: dispatch)
unit-postgresql-1: 08:31:48 WARNING unit.postgresql/1.juju-log s3-parameters:17: backups:Bucket relation-17 doesn't exist or you don't have access to it.
unit-postgresql-1: 08:31:49 INFO juju.worker.uniter.operation ran "s3-parameters-relation-changed" hook (via hook dispatching script: dispatch)
unit-postgresql-1: 08:32:08 INFO juju.worker.uniter.operation ran "database-peers-relation-changed" hook (via hook dispatching script: dispatch)
```

## Additional context

```
ubuntu@maas-bastion:/root$ jhack show-relation s3-integrator postgresql
relation data v0.6
┏━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┓
┃ relation (id: 17) ┃ s3-integrator ┃ postgresql ┃
┡━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━┩
│ type │ regular │ = │
│ interface │ s3 │ = │
│ model │ the current model │ = │
│ relation ID │ 17 │ = │
│ role │ provider │ requirer │
│ endpoint │ s3-credentials │ s3-parameters │
│ leader unit │ 1 │ 1 │
├───────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────┼──────────────────────────────────────────────────────────────────────────────────────────────┤
│ application data │ ╭──────────────────────────────────────────────────────────────────────────────────────────╮ │ ╭──────────────────────────────────────────────────────────────────────────────────────────╮ │
│ │ │ │ │ │ │ │
│ │ │ access-key admin │ │ │ bucket relation-17 │ │
│ │ │ bucket relation-17 │ │ ╰──────────────────────────────────────────────────────────────────────────────────────────╯ │
│ │ │ endpoint https://10.240.246.4:9000 │ │ │
│ │ │ path /postgresql │ │ │
│ │ │ s3-uri-style path │ │ │
│ │ │ secret-key password │ │ │
│ │ │ tls-ca-chain ["-----BEGIN │ │ │
│ │ │ CERTIFICATE-----\nMIIFkzCCA3ugAwIBAgIUU7AVlPx6DAsIVHkHEwYaR6KdqNIwDQYJK… │ │ │
│ │ │ CERTIFICATE-----", "-----BEGIN │ │ │
│ │ │ CERTIFICATE-----\nMIIEkzCCAnugAwIBAgIUZ05+9Kt3AmDhQRbBrgFfXDvxHQswDQYJK… │ │ │
│ │ │ CERTIFICATE-----"] │ │ │
│ │ ╰──────────────────────────────────────────────────────────────────────────────────────────╯ │ │
│ unit data │ ╭─ s3-integrator/1* ─╮ │ ╭─ postgresql/1* ─╮ │
│ │ │ │ │ │ │ │
│ │ ╰────────────────────╯ │ ╰─────────────────╯ │
└───────────────────┴──────────────────────────────────────────────────────────────────────────────────────────────┴──────────────────────────────────────────────────────────────────────────────────────────────┘
```

```bash
mc ls --insecure myminio/foobar/postgresql
[2025-07-14 08:31:00 UTC] 36B STANDARD model-uuid.txt
[2025-07-14 08:39:48 UTC] 0B archive/
[2025-07-14 08:39:48 UTC] 0B backup/
```

- If I use 1/stable channel of s3-integrator, postgres is also setting the relation databag, but it is simply ignored. I attach a screenshot from an experiment I conducted.

- This behavior to set the relation id in the databag is part of the s3 relation library: https://github.com/canonical/data-platform-libs/blob/main/lib/charms/data_platform_libs/v0/s3.py#L666

Reproduce the deployment using the listed Juju commands and inspect the s3-parameters and s3-credentials relation events in the log. Compare the 2/edge behavior with 1/stable, focusing on why the relation data changes the bucket to relation-17. Done means backup initialization continues using the configured foobar bucket and creates the expected PostgreSQL backup paths.

Written by the indexing model from the issue text.