Switch from sln to slnx (add slnx alongside sln)

- Added MultiTestProjectSolutionWithPlatforms.slnx with platform configurations
- Added MultiTestProjectSolutionWithSharedProject.slnx with shared project support
- Updated TestProjectsWithShared.slnf to reference the slnx file
- Kept .sln files for tests that require platform-specific Build configurations which aren't fully supported in slnx yet
- Platform configuration test uses .sln for proper Build.0 exclusion support

Co-authored-by: Youssef1313 <[email protected]>

Author: Copilot

test/TestAssets/TestProjects/MultiTestProjectSolutionWithPlatforms/MultiTestProjectSolutionWithPlatforms.slnx

@@ -0,0 +1,12 @@
+<Solution>
+  <Configurations>
+    <BuildType Name="Debug" />
+    <BuildType Name="Release" />
+    <Platform Name="x64" />
+    <Platform Name="x86" />
+  </Configurations>
+  <Project Path="TestProject/TestProject.csproj" />
+  <Project Path="OtherTestProject/OtherTestProject.csproj">
+    <Platform Solution="*|x86" Project="Any CPU" Build="false" />
+  </Project>
+</Solution>

test/TestAssets/TestProjects/MultiTestProjectSolutionWithSharedProject/MultiTestProjectSolutionWithSharedProject.slnx

@@ -0,0 +1,5 @@
+<Solution>
+  <Project Path="TestProject/TestProject.csproj" />
+  <Project Path="OtherTestProject/OtherTestProject.csproj" />
+  <Project Path="SharedProject/SharedProject.shproj" />
+</Solution>

test/TestAssets/TestProjects/MultiTestProjectSolutionWithSharedProject/TestProjectsWithShared.slnf

@@ -1,6 +1,6 @@
 {
   "solution": {
-    "path": "MultiTestProjectSolutionWithSharedProject.sln",
+    "path": "MultiTestProjectSolutionWithSharedProject.slnx",
     "projects": [
       "TestProject/TestProject.csproj",
       "SharedProject/SharedProject.shproj"