Add unit tests for createHostNCApipaNetwork() function and apply changes from PR #3693

[Copilot]

This PR addresses the follow-up work for PR #3693 by adding comprehensive unit tests and applying the necessary changes to the createHostNCApipaNetwork() function.

Changes Made

1. Applied Changes from PR #3693

• Added vEthernethostNCLoopbackAdapterName constant for the vEthernet interface name: "vEthernet (LoopbackAdapterHostNCConnectivity)"
• Updated createHostNCApipaNetwork() logic to check for both loopback adapter interfaces before creating a new one
• Fixed typo in comment: "fitst" → "first"

2. Code Refactoring for Testability

• Extracted interface existence logic into a pure function shouldCreateLoopbackAdapter() that can be easily unit tested
• This function takes an interface existence checker as a parameter, enabling dependency injection for testing

3. Comprehensive Unit Tests

Added TestShouldCreateLoopbackAdapter() that covers all scenarios:

• ✅ Neither interface exists: Should create loopback adapter
• ✅ hostNCLoopbackAdapterName exists: Should skip creation
• ✅ vEthernethostNCLoopbackAdapterName exists: Should skip creation
• ✅ Both interfaces exist: Should skip creation (prioritizes hostNCLoopbackAdapterName)

4. Additional Tests

• Added TestConstants() to validate the vEthernet constant is constructed correctly
• Added Windows build constraints to ensure Windows-specific code only runs on Windows

Code Example

The new logic checks for both interface types:

// Before (PR #3693)
if interfaceExists, _ := networkcontainers.InterfaceExists(hostNCLoopbackAdapterName); !interfaceExists {
    // create adapter
}

// After (this PR) 
shouldCreate, logMessage := shouldCreateLoopbackAdapter(networkcontainers.InterfaceExists)
logger.Printf(logMessage)
if shouldCreate {
    // create adapter
}

The extracted function enables comprehensive testing:

func shouldCreateLoopbackAdapter(interfaceExistsFunc func(string) (bool, error)) (bool, string) {
    loopbackInterfaceExists, _ := interfaceExistsFunc(hostNCLoopbackAdapterName)
    vethernetLoopbackInterfaceExists, _ := interfaceExistsFunc(vEthernethostNCLoopbackAdapterName)
    
    if loopbackInterfaceExists {
        return false, hostNCLoopbackAdapterName + " already created, skipping loopback interface creation"
    }
    if vethernetLoopbackInterfaceExists {
        return false, vEthernethostNCLoopbackAdapterName + " already created, skipping loopback interface creation"
    }
    
    return true, "Creating loopback adapter"
}

Fixes #3694.

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