Add CI workflow to run C# GSDK unit tests on PRs and update projects to net10.0

[Copilot]

What this PR does / why we need it:

Adds a GitHub Actions workflow that runs C# GSDK unit tests on pull requests when csharp/** files are modified, and updates C# application/test projects from EOL .NET versions to .NET 10.

• Sets up .NET 10.0 SDK in the CI workflow
• Builds GSDK_CSharp_Standard.sln, then runs dotnet test on csharp/Tests/Tests.csproj
• Also triggers on changes to the workflow file itself
• Scopes GITHUB_TOKEN to contents: read
• Updates Tests/Tests.csproj from net6.0 to net10.0
• Updates WinTestRunnerGame/WinTestRunnerGame.csproj from net8.0 to net10.0
• Updates WindowsTestAppCSharp/WindowsTestAppCSharp.csproj from netcoreapp2.1 to net10.0
• Keeps GSDK_CSharp_Standard.csproj on netstandard2.0 since it's a NuGet library package requiring broad consumer compatibility

Special notes for your reviewer:

All 17 existing unit tests pass on .NET 10. The GSDK library itself remains on netstandard2.0 to maintain compatibility for NuGet consumers across different .NET versions.

If applicable:

• this PR contains documentation
• this PR contains unit tests
• this PR has been tested for backwards compatibility

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[dgkanatsios]

@copilot .NET 6 is EOL, can you update C# projects to net10?

[Copilot]

@copilot .NET 6 is EOL, can you update C# projects to net10?

Updated all C# application/test projects to net10.0 and the workflow to use .NET 10 SDK in commit 8a4fcb1. The GSDK library itself remains on netstandard2.0 since it's a NuGet package that needs broad consumer compatibility.

Updated projects:

• Tests/Tests.csproj: net6.0 → net10.0
• WinTestRunnerGame/WinTestRunnerGame.csproj: net8.0 → net10.0
• WindowsTestAppCSharp/WindowsTestAppCSharp.csproj: netcoreapp2.1 → net10.0

All 17 tests pass on .NET 10.