feat: remove jump to swift-postrouting in iptables legacy as rules already exist in iptables nftables

[QxBytes]

Reason for Change:

Contains same changes as #3930 but without the behavioral change of snat to node ip

Deletes iptables legacy jump to swift postrouting rule before adding nftables rules because:
If we delete before:
If iptables maps to iptables nftables, we should already have nftables rules programmed on the node because cns 1.7.1 has fully rolled out, so deleting first should not cause any connectivity blip.
If iptables somehow maps to iptables legacy, we have a blip and then write the rule immediately after

If we delete afterwards:
If iptables maps to iptables nftables, we write the rules and delete the legacy jump after as expected
If iptables somehow maps to iptables legacy, we would add the rules and then delete the jump we just added, breaking the node until the cns restarts

Issue Fixed:

Requirements:

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

Notes:
Tested upgrade from 1.7.0 --> 1.7.1 --> 1.7.2 (this PR) for cns

[Copilot]

Pull Request Overview

This PR removes the jump to swift-postrouting in iptables legacy since the same rules already exist in iptables nftables. It introduces an iptables legacy client interface to clean up legacy rules during SNAT rule programming.

• Adds a new iptablesLegacyClient interface for deleting legacy iptables rules
• Implements legacy iptables cleanup in SNAT rule programming
• Updates logging to be more descriptive about SWIFT-POSTROUTING chain 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 client for Windows
cns/restserver/internalapi_linux.go	Implements legacy iptables deletion and integrates cleanup into SNAT programming
cns/restserver/internalapi_linux_test.go	Adds test coverage for legacy iptables deletion
cns/fakes/iptablesfake.go	Adds mock implementation for legacy iptables client

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[QxBytes]

/azp run Azure Container Networking PR

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).