fix(agent): allow DHCP on isolated sites without gateway or DNS forwarders

[ArshSSandhu]

Summary

• Allow dnsmasq configuration to be applied when the site has an internal domain, DHCP range, and subnet mask, without requiring a gateway or DNS forwarders.
• Suppress the DHCP router option when no gateway is configured.
• Allow the dnsmasq servers file to remain empty when no DNS forwarders are configured.

Testing

• npm ci
• npm run build
• git diff --check
• End-to-end DHCP validation on an isolated Proxmox site remains to be completed.

Related Issue

Fixes #455

[Copilot]

Pull request overview

This PR updates the agent’s dnsmasq rendering logic to support “isolated” sites (air‑gapped/no upstream network) by treating gateway and DNS forwarders as optional while still applying DHCP + internal DNS config when core DHCP settings are present.

Changes:

• Relax dnsmasq render preconditions to require only internalDomain, dhcpRange, and subnetMask.
• Allow an empty dnsmasq servers file when no DNS forwarders are configured.
• Adjust DHCP router option handling for sites without a gateway (but see review comment).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
agent/src/apply.ts	Loosens dnsmasq “render/skip” gating to enable isolated-site DHCP/internal DNS.
agent/templates/dnsmasq/servers.ejs	Makes DNS forwarders optional and safely renders an empty servers file when absent.
agent/templates/dnsmasq/dhcp-opts.ejs	Attempts to handle missing gateway for DHCP router option (needs adjustment per comment).

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