[Bug]: Network is unreachable after turning on VPN

### I have done the following

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

### Steps to reproduce

Reproduce:
1. container system start
2. container start -ai ubuntu
3. open and connect to VPN on MAC host

### Current behavior

then the client ubuntu container can no longer connect to internet

### Expected behavior

then the client ubuntu container should connect to internet stably

### Environment

```markdown
- OS: Tahoe 26.2
- Xcode: xcode-select version 2416
- Container: container CLI version 0.9.0-35-g899081d (build: release, commit: 899081d)
```

### Relevant log output

```shell
N/A
```

### Code of Conduct

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

Start by reproducing the issue with `container system start`, an interactive Ubuntu container, and a connected VPN on the macOS host. No source file or test is named, so trace the container networking behavior during VPN connection and compare it with the expected stable internet access. Done means the Ubuntu container remains able to connect to the internet after the VPN is enabled.

Written by the indexing model from the issue text.