[Bug]: container system stop / container system start causes a new default network

### I have done the following

- [x] I have searched the existing issues
- [ ] If possible, I've reproduced the issue using the 'main' branch of this project

### Steps to reproduce

I had a VM:
```
$ container list
ID IMAGE OS ARCH STATE ADDR CPUS MEMORY
buildkit ghcr.io/apple/container-builder-shim/builder:0.6.1 linux arm64 running 192.168.66.4 2 2048 MB
```

On stop it hung and had to be killed via `Virtual Machine Service for container-runtime-linux` is killed via Activity Monitor; but for reproduction just kill the container and then network mask will change:

```nomad
$ container system stop
$ container system start
$ container network list
NETWORK STATE SUBNET
default running 192.168.67.0/24
```

### Current behavior

A new subnet is generated each time this happens.

### Expected behavior

The subnet to remain as before; continue to use `192.168.66.0/24`

### Environment

```markdown
- OS: 26.0.1 (25A362)
- Xcode:
- Container: container CLI version 0.6.0 (build: release, commit: a23bcf0)
```

### Relevant log output

```shell
2025-11-03 14:41:49.891895+0200 0x5d68076 Info 0x0 45689 0 container-apiserver: [com.apple.container:APIServer] Registering plugin [id=com.apple.container.container-network-vmnet.default]
2025-11-03 14:41:49.950327+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] starting container-network-vmnet [id=default]
2025-11-03 14:41:49.950331+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] configuring XPC server [id=default]
2025-11-03 14:41:49.950673+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] creating vmnet network [id=default]
2025-11-03 14:41:49.950673+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] created vmnet network [id=default]
2025-11-03 14:41:49.950760+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] starting vmnet network [id=default] [mode=nat]
2025-11-03 14:41:49.956032+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] started vmnet network [id=default] [cidr=192.168.67.0/24] [mode=nat]
2025-11-03 14:41:49.956083+0200 0x5d6808e Info 0x0 45704 0 container-network-vmnet: [com.apple.container:NetworkVmnetHelper] starting XPC server [id=default]
```

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct