fix(vm): make virtualMachineCIDRs optional

[diafour]

Description

Allow ModuleConfig without spec.settings.virtualMachineCIDRs.

What changed:
- Make virtualMachineCIDRs optional in OpenAPI schema.
- Stop blocking module enablement when CIDRs are not configured.
  - Make ModuleConfig CIDR parsing safe when the field is absent.
  - Skip CIDR overlap checks when virtualMachineCIDRs is not set.
- New behavior when CIDRs are not configured:
  - Reject VMs with explicit Main network.
  - Reject VMIP usage and VirtualMachineIPAddress resources.
- Validate changing ModuleConfig: protect removing CIDRs if VMIP leases are present.

Also updated docs and tests.

Why do we need it, and what problem does it solve?

What is the expected result?

Checklist

• The code is covered by unit tests.
• e2e tests passed.
• Documentation updated according to the changes.
• Changes were tested in the Kubernetes cluster manually.

Changelog entries

section: vm
type: fix
summary: "Allow VM creation when ModuleConfig configured without spec.settings.virtualMachineCIDRs."