Migrates solution file to SLNX format

Replaces the traditional Visual Studio .sln file with the new .slnx solution file format and updates all related references in build scripts and GitHub workflows.

Author: Keboo

.github/copilot-instructions.md

@@ -41,13 +41,13 @@ The MaterialDesignInXamlToolkit is a **theme library** for WPF applications that
 ### Build and Test
 ```powershell
 # Restore dependencies
-dotnet restore MaterialDesignToolkit.Full.sln
+dotnet restore MaterialDesignToolkit.Full.slnx
 
 # Build (requires Windows)
-dotnet build MaterialDesignToolkit.Full.sln --configuration Release --no-restore -p:Platform="Any CPU" -p:TreatWarningsAsErrors=True
+dotnet build MaterialDesignToolkit.Full.slnx --configuration Release --no-restore -p:Platform="Any CPU" -p:TreatWarningsAsErrors=True
 
 # Run tests
-dotnet test MaterialDesignToolkit.Full.sln --configuration Release --no-build
+dotnet test MaterialDesignToolkit.Full.slnx --configuration Release --no-build
 
 # Build NuGet packages
 .\build\BuildNugets.ps1 -MDIXVersion "x.x.x" -MDIXColorsVersion "x.x.x" -MDIXMahAppsVersion "x.x.x"

.github/workflows/build_artifacts.yml

@@ -23,7 +23,7 @@ jobs:
     runs-on: windows-latest
 
     env:
-      solution: MaterialDesignToolkit.Full.sln
+      solution: MaterialDesignToolkit.Full.slnx
 
         steps:
             - uses: actions/checkout@v6

.github/workflows/release.yml

@@ -9,7 +9,7 @@ on:
                 type: string
 
 env:
-    solution: MaterialDesignToolkit.Full.sln
+    solution: MaterialDesignToolkit.Full.slnx
     GITHUB_TOKEN: ${{ secrets.SA_PAT }}
 
 defaults: