[WSLc] CI failure: directional network policy isn't treated as immutable

### Relevant area(s)

Windows

### Brief description of your issue

Affected OS versions: Windows 23H2, 24H2, 25H2, Pre-release.

Run: https://github.com/microsoft/mxc/actions/runs/35198276932/job/105131096734
Failure (line 323 - 326):
```
[D: exec (directional network change rejected structurally)]
PASS: exit code is non-zero (contract rejected)
FAIL: error.code is 'malformed_request' (got 'policy_validation')
FAIL: error.message identifies immutable exec network policy (got 'network mode is bound to the provision phase and cannot be changed by the WSLc backend after provisioning')
```

### Steps to reproduce

See above

### Expected behavior

See above

### Actual behavior

See above