Migrate XHarness solution to .slnx format (#1432)

* Initial plan

* Migrate XHarness.sln to .slnx format and update documentation

Co-authored-by: akoeplinger <1376924+akoeplinger@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: akoeplinger <1376924+akoeplinger@users.noreply.github.com>

Author: Copilot

.github/copilot-instructions.md

@@ -64,7 +64,7 @@ xharness [platform] [command] [options]
 
 ### Build System
 - Use `./build.sh` (Linux/macOS) or `Build.cmd` (Windows) for proper SDK setup
-- Alternative: `dotnet build XHarness.sln` (requires correct .NET version)
+- Alternative: `dotnet build XHarness.slnx` (requires correct .NET version)
 - Integration with Arcade SDK for .NET Foundation build standards
 - Azure DevOps pipelines for CI/CD
 
@@ -157,7 +157,7 @@ XHarness uses standardized exit codes (see `src/Microsoft.DotNet.XHarness.Common
 ├── eng/                          # Build and engineering files
 ├── tools/                        # Development tools and scripts
 ├── azure-pipelines*.yml         # CI/CD pipeline definitions
-├── XHarness.sln                 # Main solution file
+├── XHarness.slnx                # Main solution file
 ├── build.sh / Build.cmd         # Build scripts
 └── README.md                    # Main documentation
 ```

README.md

@@ -152,12 +152,12 @@ Currently we support Xunit and NUnit test assemblies but the `Microsoft.DotNet.X
 When working on XHarness, there are couple of neat hacks that can improve the inner loop.
 The repository can either be built using regular .NET, assuming you have new enough version:
 ```
-dotnet build XHarness.sln
+dotnet build XHarness.slnx
 ```
 or you can use the build scripts `build.sh` or `Build.cmd` in repository root which will install the correct .NET SDK into the `.dotnet` folder.
 You can then use
 ```
-./.dotnet/dotnet build XHarness.sln
+./.dotnet/dotnet build XHarness.slnx
 ```
 
 You can also use Visual Studio 2019+ and just F5 the `Microsoft.DotNet.XHarness.CLI` project.

XHarness.sln

@@ -0,0 +1,25 @@
+<Solution>
+  <Folder Name="/Solution Items/">
+    <File Path=".editorconfig" />
+    <File Path="global.json" />
+  </Folder>
+  <Folder Name="/src/">
+    <Project Path="src/Microsoft.DotNet.XHarness.Android/Microsoft.DotNet.XHarness.Android.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.Apple/Microsoft.DotNet.XHarness.Apple.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.CLI/Microsoft.DotNet.XHarness.CLI.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.Common/Microsoft.DotNet.XHarness.Common.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.InstrumentationBase.Xunit/Microsoft.DotNet.XHarness.DefaultAndroidEntryPoint.Xunit.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.iOS.Shared/Microsoft.DotNet.XHarness.iOS.Shared.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.TestRunners.Common/Microsoft.DotNet.XHarness.TestRunners.Common.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.TestRunners.NUnit/Microsoft.DotNet.XHarness.TestRunners.NUnit.csproj" />
+    <Project Path="src/Microsoft.DotNet.XHarness.TestRunners.Xunit/Microsoft.DotNet.XHarness.TestRunners.Xunit.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/Microsoft.DotNet.XHarness.Android.Tests/Microsoft.DotNet.XHarness.Android.Tests.csproj" />
+    <Project Path="tests/Microsoft.DotNet.XHarness.Apple.Tests/Microsoft.DotNet.XHarness.Apple.Tests.csproj" />
+    <Project Path="tests/Microsoft.DotNet.XHarness.CLI.Tests/Microsoft.DotNet.XHarness.CLI.Tests.csproj" />
+    <Project Path="tests/Microsoft.DotNet.XHarness.Common.Tests/Microsoft.DotNet.XHarness.Common.Tests.csproj" />
+    <Project Path="tests/Microsoft.DotNet.XHarness.iOS.Shared.Tests/Microsoft.DotNet.XHarness.iOS.Shared.Tests.csproj" />
+    <Project Path="tests/Microsoft.DotNet.XHarness.TestRunners.Tests/Microsoft.DotNet.XHarness.TestRunners.Tests.csproj" />
+  </Folder>
+</Solution>