feat: snat azure dns traffic to node ip in cns linux

[QxBytes]

Reason for Change:

Changes the ip CNS-added IPTables rules SNAT to from the primary ip to node ip for linux podsubnet scenarios (both azure and cilium cases). CNI-added iptables rules are not modified and windows behavior remains the same (will be modified in a future PR).

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• relevant PR labels added

Notes:
Tested:

• Podsubnet linux upgrade from 1.7.3 --> this version --> downgrade to 1.7.3 and dns snat is as expected
• Podsubnet linux cilium ugprade from 1.7.3 --> this version --> downgrade to 1.7.3 and dns snat is as expected

[Copilot]

Pull Request Overview

This PR modifies SNAT behavior for Azure DNS traffic in Linux podsubnet scenarios by changing the source IP from the primary subnet IP to the node IP, and removes conflicting iptables-legacy rules to prevent conflicts with iptables-nftables.

• Changes SNAT target from subnet primary IP to node IP for Azure DNS traffic
• Removes jump to SWIFT-POSTROUTING in iptables-legacy to avoid rule conflicts
• Adds support for iptables-legacy client interface to handle cleanup operations

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
cns/restserver/restserver.go	Adds iptablesLegacyClient interface and getter method
cns/restserver/internalapi_windows.go	Implements unsupported legacy iptables for Windows
cns/restserver/internalapi_linux_test.go	Updates tests to verify node IP usage and legacy rule deletion
cns/restserver/internalapi_linux.go	Implements legacy iptables deletion and changes SNAT target to node IP
cns/fakes/iptablesfake.go	Adds mock implementation for legacy iptables testing

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.
You can also share your feedback on Copilot code review for a chance to win a $100 gift card. Take the survey.}

[QxBytes]

/azp run Azure Container Networking PR

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[github-actions]

This pull request is stale because it has been open for 2 weeks with no activity. Remove stale label or comment or this will be closed in 7 days

[github-actions]

Pull request closed due to inactivity.

[QxBytes]

/azp run Azure Container Networking PR

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).