Failed to Move VM to Another Account

### problem

Up on stopping an Instance and trying to move it to another account...

...it does not make clear what caused the failure.

Even as a Root Admin, there is no explanation of what prevented the move from happening.

By looking at the logs, the message is clear but it is followed by an unhandled exception.

Sanitized fragments of the logs:

```
2026-06-11 20:07:53,953 ERROR [o.a.c.a.c.a.v.AssignVMCmd] (qtp1404565079-58786:[ctx-e08ee8a2, ctx-7b35ebda]) (logid:46523a5c) Failed to move VM [4944]. com.cloud.exception.InvalidParameterValueException: Snapshots exist for volume [ROOT-4944]. Detach volume or remove snapshots for the volume before assigning VM to another user.
at com.cloud.vm.UserVmManagerImpl.validateIfVolumesHaveNoSnapshots(UserVmManagerImpl.java:7961)
at com.cloud.vm.UserVmManagerImpl.moveVmToUser(UserVmManagerImpl.java:7871)
```

```
2026-06-11 19:58:58,841 ERROR [o.a.c.a.c.a.v.AssignVMCmd] (qtp1404565079-58786:[ctx-b7b9a764, ctx-86b9f79a]) (logid:1096b7f6) Failed to move VM [4944]. com.cloud.exception.InvalidParameterValueException: Remove any Port Forwarding rules for VM [VM instance {"id":4944,"instanceName":"i-73-4944-VM","state":"Stopped","type":"User","uuid":"*"}] before assigning it to another user.
at com.cloud.vm.UserVmManagerImpl.validateIfVmHasNoRules(UserVmManagerImpl.java:7934)
at com.cloud.vm.UserVmManagerImpl.moveVmToUser(UserVmManagerImpl.java:7868)
```

### versions

ACS 4.22.0.1

### The steps to reproduce the bug

1.
2.
3.
...

### What to do about it?

_No response_

從 AssignVMCmd 和 UserVmManagerImpl 開始，特別檢視 validateIfVolumesHaveNoSnapshots、validateIfVmHasNoRules 和 moveVmToUser；然後審查關聯的 pull request #13675。完成標準應包括：對於移動失敗，提供清楚的面向使用者說明，而不是拋出未處理的例外。

由索引模型根據 Issue 內容生成。