fix: Do not include router option if no nat

[MMaiero]

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[pierantoniomerlino]

@MMaiero I verified the issue on my device and that this PR fixes it. I tested the solution with dhcpd and dnsmasq. I noticed that dhcpd had been already configured to not add the router address if in DHCP Only.

However, it seems that if the framework is configured in DHCP Only and the PassDNS is set to true, the DNS are indeed passed to the client:

I don't think that this is a correct configuration. I know that it's bit out of scope...

[Copilot]

PR Overview

This pull request ensures that the router option is omitted in the DHCP configuration when NAT is disabled. Key changes include updates to test cases to cover NAT enabled/disabled scenarios, modifications in the DHCP server configuration writer to conditionally set the router, and adjustments in the DHCP config converters (Dhcpd, Dnsmasq, and Udhcpd) to properly handle the absence of a router address.

Reviewed Changes

File	Description
kura/test/org.eclipse.kura.nm.test/src/test/java/org/eclipse/kura/nm/configuration/writer/DhcpServerConfigWriterTest.java	Added test cases to cover scenarios with NAT disabled.
kura/org.eclipse.kura.nm/src/main/java/org/eclipse/kura/nm/configuration/writer/DhcpServerConfigWriter.java	Updated to conditionally set the router field based on the NAT flag.
kura/org.eclipse.kura.linux.net/src/main/java/org/eclipse/kura/linux/net/dhcp/server/DhcpdConfigConverter.java	Refactored configuration appending by splitting DNS, interface, router, lease times, and pool range into dedicated methods.
kura/test/org.eclipse.kura.linux.net.test/src/test/java/org/eclipse/kura/linux/net/dhcp/server/DhcpdConfigConverterTest.java	Updated expected configurations for NAT disabled scenarios.
kura/org.eclipse.kura.linux.net/src/main/java/org/eclipse/kura/linux/net/dhcp/server/DnsmasqConfigConverter.java	Adjusted the router option handling to conditionally include the router value.
kura/org.eclipse.kura.linux.net/src/main/java/org/eclipse/kura/linux/net/dhcp/server/UdhcpdConfigConverter.java	Modified to conditionally output the router line based on its availability.

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

[Copilot]

When 'nat.enabled' is false and config.getRouterAddress() is null, this line produces a router option output like 'dhcp-option=eth0,3' without a value. To adhere to the PR intent of omitting the router option entirely, consider conditionally skipping the entire router option line when routerAddress is null.

Suggested change

	String routerAddress = isNull(config.getRouterAddress()) ? "" : "," + config.getRouterAddress().toString();
	sb.append(DHCP_OPTION_KEY).append(config.getInterfaceName()).append(",3").append(routerAddress).append("\n");
	if (!isNull(config.getRouterAddress())) {
	String routerAddress = "," + config.getRouterAddress().toString();
	sb.append(DHCP_OPTION_KEY).append(config.getInterfaceName()).append(",3").append(routerAddress).append("\n");
	}