Former primary stuck at 1/2 after failover due to barman-cloud-check-wal-archive "Expected empty archive"

### Description

After a failover/switchover on a running cluster (bootstrapped with `initdb`, **not** a recovery/restore), the `plugin-barman-cloud` sidecar on replica pods keeps failing with `"WAL archive check failed: Expected empty archive"`. This prevents the replica from becoming fully ready (1/2 containers).

The root cause is that `barman-cloud-check-wal-archive` is executed on **every WAL Archive gRPC call**, including on **replica pods**, and it fails because the S3 bucket already contains WAL files from previous timelines (which is expected after a switchover).

### Environment

- **CloudNativePG operator**: v1.28.1
- **plugin-barman-cloud**: v0.11.0
- **PostgreSQL**: 18.3 (`ghcr.io/cloudnative-pg/postgresql:18.3`)
- **Kubernetes**: v1.31
- **Object storage**: Ceph RGW (S3-compatible, via Rook)

### Cluster configuration

The cluster is bootstrapped with `initdb` (no recovery, no externalClusters):

```yaml
spec:
instances: 3
bootstrap:
initdb:
database: system
encoding: UTF8
owner: app
plugins:
- name: barman-cloud.cloudnative-pg.io
enabled: true
isWALArchiver: true
parameters:
barmanObjectName: my-object-store
```

### Steps to reproduce

1. Create a 3-instance CNPG cluster with `initdb` bootstrap and `plugin-barman-cloud` with `isWALArchiver: true`
2. Wait for all 3 pods to be 2/2 Running
3. A failover or switchover occurs (automatic or manual), changing the timeline (e.g., timeline 1 → 2 → 3)
4. After the failover, one or more replica pods get stuck at 1/2 — the `plugin-barman-cloud` sidecar blocks WAL archiving

### What happens

After the switchover, the former primary is demoted to replica. The instance manager detects leftover WAL files and explicitly triggers archiving on the demoted pod:

```
{"msg":"Detected ready WAL files in a former primary, triggering WAL archiving"}
```

This causes the `plugin-barman-cloud` sidecar to attempt WAL archiving. However, the S3 bucket legitimately contains WAL files from previous timelines (archived by this same pod when it was primary). The plugin executes `barman-cloud-check-wal-archive`, finds the bucket is not empty, and fails:

```
barman-cloud-check-wal-archive: ERROR: WAL archive check failed for server : Expected empty archive
```

plugin-barman-cloud sidecar log (replica pod):

```
{"level":"info","msg":"barman-cloud-check-wal-archive checking the first wal"}
{"level":"info","logger":"barman-cloud-check-wal-archive","msg":"ERROR: WAL archive check failed for server : Expected empty archive","pipe":"stderr"}
{"level":"error","msg":"Error invoking barman-cloud-check-wal-archive",
"options":["--endpoint-url","http://","--cloud-provider","aws-s3","s3://",""],
"exitCode":-1,"error":"exit status 1"}
```

### Expected behavior
All the cluster pods are correctly running after a switchover/failover.

Tái hiện sự cố với một cluster initdb gồm ba instance, bật plugin plugin-barman-cloud và isWALArchiver, sau đó theo dõi lệnh gọi WAL Archive gRPC và hành vi của barman-cloud-check-wal-archive trên Primary trước đó. Xác minh đường dẫn Failover trong đó một pod bị hạ cấp kích hoạt việc lưu trữ mặc dù đã tồn tại các tệp WAL từ những timeline trước. Hoàn tất khi tất cả pod trở lại 2/2 sau một Switchover hoặc Failover mà không có lỗi Expected empty archive.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.