Restructure C# samples: unified csproj with OS detection

- Flatten dual windows/cross-platform csproj into single unified csproj per sample
- MSBuild IsOSPlatform condition auto-selects WinML on Windows, standard SDK elsewhere
- Move from GettingStarted/src/{Sample}/ to samples/cs/{sample-dir}/
- Each sample gets its own .sln file
- Rename HelloFoundryLocalSdk to native-chat-completions (aligns with docs)
- Remove old GettingStarted directory (windows/, cross-platform/, old .sln files)
- Update CI workflow to remove Windows/cross-platform skip logic
- Shared/Utils.cs referenced via Compile Include from 6 non-tutorial samples
- 4 tutorials get WinML support (previously cross-platform only)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: samuel100

.github/workflows/samples-integration-test.yml

@@ -194,12 +194,6 @@ jobs:
           $projects = Get-ChildItem samples/cs -Recurse -Filter "*.csproj"
           foreach ($proj in $projects) {
             $name = $proj.BaseName
-            $relPath = $proj.FullName.Replace((Get-Location).Path, '').TrimStart('\','/')
-            # Skip windows-only projects on non-Windows platforms
-            if ($relPath -match '[/\\]windows[/\\]' -and -not $IsWindows) {
-              Write-Host "`n=== Skipping (Windows-only): $name ==="
-              continue
-            }
             Write-Host "`n=== Building: $name ==="
             dotnet build $proj.FullName --configuration Debug 2>&1
             if ($LASTEXITCODE -ne 0) {