refactor: migrate away from .shproj to SDK-style linked files

- Replace .shproj/.projitems Import with direct Compile Include pattern
- Update Roslyn4120 and Roslyn480 projects to link files using wildcard pattern
- Remove shared project references from solution file
- Fix using directives ordering in TestHelper.cs (StyleCop compliance)

This makes the build dotnet CLI-friendly and works on all CI agents without
requiring Visual Studio-specific MSBuild extensions.

All 33 tests pass.

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

Author: Copilot

src/ReactiveUI.SourceGenerator.Tests/TestHelper.cs

@@ -10,15 +10,15 @@
 using NuGet.LibraryModel;
 using NuGet.Versioning;
 
+using NUnit.Framework;
+
 using ReactiveMarbles.NuGet.Helpers;
 
 using ReactiveMarbles.SourceGenerator.TestNuGetHelper.Compilation;
 
 using ReactiveUI.SourceGenerators;
 using ReactiveUI.SourceGenerators.WinForms;
 
-using NUnit.Framework;
-
 namespace ReactiveUI.SourceGenerator.Tests;
 
 /// <summary>

src/ReactiveUI.SourceGenerators.Roslyn4120/ReactiveUI.SourceGenerators.Roslyn4120.csproj

@@ -29,6 +29,8 @@
     <InternalsVisibleTo Include="ReactiveUI.CodeFixes" />
   </ItemGroup>
 
-  <Import Project="..\ReactiveUI.SourceGenerators.Roslyn\ReactiveUI.SourceGenerators.Roslyn.projitems" Label="Shared" />
+  <ItemGroup>
+    <Compile Include="..\ReactiveUI.SourceGenerators.Roslyn\**\*.cs" LinkBase="Shared" />
+  </ItemGroup>
 
 </Project>

src/ReactiveUI.SourceGenerators.Roslyn480/ReactiveUI.SourceGenerators.Roslyn480.csproj

@@ -24,6 +24,8 @@
     <InternalsVisibleTo Include="ReactiveUI.CodeFixes" />
   </ItemGroup>
 
-  <Import Project="..\ReactiveUI.SourceGenerators.Roslyn\ReactiveUI.SourceGenerators.Roslyn.projitems" Label="Shared" />
+  <ItemGroup>
+    <Compile Include="..\ReactiveUI.SourceGenerators.Roslyn\**\*.cs" LinkBase="Shared" />
+  </ItemGroup>
 
 </Project>

src/ReactiveUI.SourceGenerators.sln

@@ -34,8 +34,6 @@ Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ReactiveUI.SourceGenerators
 EndProject
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "NestedTest", "NestedTest", "{CAFBD27B-5078-4A0C-A4E9-19DCF2A7DF16}"
 EndProject
-Project("{D954291E-2A0B-460D-934E-DC6B0785DB48}") = "ReactiveUI.SourceGenerators.Roslyn", "ReactiveUI.SourceGenerators.Roslyn\ReactiveUI.SourceGenerators.Roslyn.shproj", "{BC460DAF-92ED-41D9-9EB4-2C92121D62A8}"
-EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ReactiveUI.SourceGenerators.Roslyn480", "ReactiveUI.SourceGenerators.Roslyn480\ReactiveUI.SourceGenerators.Roslyn480.csproj", "{7C308972-5697-464F-BEAE-02FE9E03BECE}"
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "ReactiveUI.SourceGenerators.Roslyn4120", "ReactiveUI.SourceGenerators.Roslyn4120\ReactiveUI.SourceGenerators.Roslyn4120.csproj", "{BF121262-7F30-4EC0-9F03-324AC3B834B8}"
@@ -101,9 +99,4 @@ Global
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {173F891B-86A2-4226-B563-A7318CE0E2EC}
 	EndGlobalSection
-	GlobalSection(SharedMSBuildProjectFiles) = preSolution
-		ReactiveUI.SourceGenerators.Roslyn\ReactiveUI.SourceGenerators.Roslyn.projitems*{7c308972-5697-464f-beae-02fe9e03bece}*SharedItemsImports = 5
-		ReactiveUI.SourceGenerators.Roslyn\ReactiveUI.SourceGenerators.Roslyn.projitems*{bc460daf-92ed-41d9-9eb4-2c92121d62a8}*SharedItemsImports = 13
-		ReactiveUI.SourceGenerators.Roslyn\ReactiveUI.SourceGenerators.Roslyn.projitems*{bf121262-7f30-4ec0-9f03-324ac3b834b8}*SharedItemsImports = 5
-	EndGlobalSection
 EndGlobal