Fix all warnings in C# workflow

[konard]

Summary

This PR fixes all the warnings identified in issue #143 that were appearing in the C# workflow:

• Fixed CS8600 null warnings in InterfacesTests.cs by making interface variables nullable (10 warnings eliminated)
• Fixed deprecated set-output command warning by updating to use GITHUB_OUTPUT environment file instead
• Fixed NU5048 PackageIconUrl deprecation warning by switching to modern PackageIcon approach with local icon file
• Updated all actions/checkout versions from v1/v3 to v4 for consistency and to eliminate deprecation warnings
• Fixed typos in workflow job names (pusnToNuget → pushToNuget, publiseRelease → publishRelease)

Test Plan

• Local build succeeds without CS8600 warnings (dotnet test passes cleanly)
• NuGet pack succeeds without NU5048 warning (PackageIconUrl deprecation eliminated)
• Workflow syntax is valid and uses modern GitHub Actions patterns
• All changes are non-breaking and maintain existing functionality

Changes Made

1. Fixed Nullable Reference Type Warnings

File: csharp/Platform.Interfaces.Tests/InterfacesTests.cs

• Made all interface variables nullable by adding ? operator
• Eliminated all 10 CS8600 warnings about null assignments to non-nullable types

2. Updated GitHub Actions Workflow

File: .github/workflows/csharp.yml

• Updated deprecated set-output command to use GITHUB_OUTPUT environment file
• Updated all actions/checkout@v1 and actions/checkout@v3 to actions/checkout@v4
• Fixed job name typos for better readability

3. Modernized NuGet Package Configuration

File: csharp/Platform.Interfaces/Platform.Interfaces.csproj

• Replaced deprecated PackageIconUrl with modern PackageIcon approach
• Added local icon.png file to project with proper pack configuration
• Eliminated NU5048 deprecation warning

🤖 Generated with Claude Code

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Resolves #143