…efix (#4299)

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## What this PR does

Fixes #4298.

`applyClusterPluginBackup` preserves the `serverName` already on the
Cluster so that a backup does not flip the WAL-archive prefix
mid-stream, which would split the archive across two prefixes. That rule
is right; the read it rests on is not reliable in one window.

The value was read through the manager's cached client, and
`BackupJobReconciler.SetupWithManager` registers `For(&BackupJob{})` and
nothing else, so a Cluster only reaches the cache through a lazily
started informer with no resync tied to the Cluster's own changes. An
in-place `RestoreJob` deletes the Cluster and the chart re-renders it
with `bootstrap.newServerName` seconds later, so a read served during
that window describes the object that was purged — and the source's
`serverName` is written back over the freshly rendered one.
`barman-cloud-check-wal-archive` then refuses the destination with
`Expected empty archive` and the `RestoreJob` ends on its 30-minute
deadline, which is the failure `newServerName` was added in #3988 to
prevent.

The same stale read carries a stale `UID`, and that `UID` owns the
`ObjectStore` this function applies, so the store is garbage-collected
as soon as it is written.

Reading through the dynamic client this reconciler already carries keeps
the preservation rule and makes the value it rests on truthful. There is
precedent in the same file: the HelmRelease suspend/resume path reads
that way for the same reason.

Observed on a v1.6.3 cluster, twice on separate runs with the same
four-second gap between the chart's apply and the driver's. The
regression test makes the two clients disagree the way they disagree in
that window and asserts the driver believes the live one; reverting the
fix fails it on the serverName assertion.


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- [x] No downstream repository is affected by this change
- [ ] [cozystack/website](https://github.com/cozystack/website) -
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[cozystack/terraform-provider-cozystack](https://github.com/cozystack/terraform-provider-cozystack)
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- [ ]
[cozystack/cozystack-telemetry-server](https://github.com/cozystack/cozystack-telemetry-server)
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[cozystack/external-apps-example](https://github.com/cozystack/external-apps-example)
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follow-up:
- [ ] [cozystack/community](https://github.com/cozystack/community) -
follow-up:

### Release note

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```release-note
fix(postgres): an in-place restore no longer loses its fresh WAL-archive prefix to a stale Cluster read, which left the restored cluster archiving onto the source's prefix and the RestoreJob failing on its deadline
```


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Backup processing now uses the latest CNPG Cluster state rather than
potentially outdated cached information. This ensures the cluster’s
current server name and identity are used when associating backup
ObjectStore resources, including when the cached and live cluster
details differ.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->