Optimize string concatenation in network utility methods

[Copilot]

Three network utility methods used O(n²) string concatenation in loops instead of O(n) alternatives.

Changes

• Subnetmask.ConvertCidrToSubnetmask: Replace loop concatenation with new string('1', cidr) constructor
• IPv4Address.ConvertIPAddressListToString: Replace manual loop with string.Join()
• IPv4Address.ConvertIPAddressWithSubnetmaskListToString: Replace manual loop with string.Join() + LINQ Select

Example

Before:

var bits = string.Empty;
for (var i = 0; i < cidr; i++)
    bits += "1";

After:

var bits = new string('1', cidr);

Impact is most noticeable during network interface export operations and subnet calculations with large datasets.

Original prompt

Identify and suggest improvements to slow or inefficient code

---

💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[BornToBeRoot]

@copilot remove draft

[Copilot]

@copilot remove draft

I don't have the capability to change the draft status of the pull request. You'll need to click "Ready for review" in the GitHub UI to remove the draft status.