[BUG] backupconfig/vaultconfig update not working as expected

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/main/specification/recoveryservicesbackup/resource-manager/Microsoft.RecoveryServices/stable/2025-02-01/bms.json

### API Spec version

2025-02-01

### Describe the bug

Lets say I want to update the soft delete retention period of a vault.

https://learn.microsoft.com/en-us/rest/api/backup/backup-resource-vault-configs/update?view=rest-backup-2025-02-01&tabs=HTTP#code-try-0

`PATCH https://management.azure.com/subscriptions/xxxx/resourceGroups/yyyy/providers/Microsoft.RecoveryServices/vaults/zzzz/backupconfig/vaultconfig?api-version=2025-02-01`

With body:
```
{
"properties": {
"softDeleteRetentionPeriodInDays": 18
}
}
```

Returns 200 (as in it should have succeeded)

but instead of changing anything the call does nothing.

### Expected behavior

Changing the soft delete retention period

### Actual behavior

nothing changes

### Reproduction Steps

curl or use the web page in description

### Environment

Edge / RHEL8 curl

Start with the 2025-02-01 bms.json specification and the Backup Resource Vault Configs update operation described in the issue. Reproduce the PATCH request with curl, then compare the documented request and response with the observed vault behavior. Done means identifying whether the specification needs a correction or whether the unchanged setting is a service-side issue.

Written by the indexing model from the issue text.