fix: correct DiagnosticClasses polyfill path, simplify test infrastructure

Fix shared source path for DiagnosticClasses.cs and Lock.cs polyfills.
Remove redundant test infrastructure and consolidate test files.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ANcpLua

src/Build/Common/Common.targets

@@ -358,7 +358,7 @@
     />
   </Target>
 
-  <!-- Disable packages -->
+  <!-- Disable packages (Rider XSD false positive: Remove-only Analyzer items are valid MSBuild) -->
   <Target Name="Disable_SponsorLink" BeforeTargets="CoreCompile" Condition="'$(Disable_SponsorLink)' != 'false'">
     <ItemGroup>
       <Analyzer Remove="@(Analyzer)" Condition="'%(Analyzer.Filename)' == 'DevLooped.SponsorLink'"/>

src/Build/Common/LegacySupport.targets

@@ -178,7 +178,7 @@
        StackTraceHiddenAttribute
        ───────────────────────────────────────────────────────────────────────── -->
   <ItemGroup Condition="'$(InjectDiagnosticClassesOnLegacy)' == 'true' AND ('$(TargetFramework)' == 'netstandard2.0' OR '$(TargetFramework)' == 'net472')">
-    <Compile Include="$(_SharedSourcePath)Polyfills/DiagnosticClasses.cs" LinkBase="Shared/Polyfills" InProject="false"/>
+    <Compile Include="$(_SharedSourcePath)MSBuild/Polyfills/DiagnosticClasses.cs" LinkBase="Shared/Polyfills" InProject="false"/>
   </ItemGroup>
 
   <ItemGroup Condition="('$(InjectStackTraceHiddenOnLegacy)' == 'true' OR '$(InjectANcpLuaDiagnosticsPolyfills)' == 'true') AND '$(_NeedsNet6Polyfills)' == 'true'">
@@ -242,7 +242,7 @@
        POLYFILLS - Lock & TimeProvider
        ───────────────────────────────────────────────────────────────────────── -->
   <ItemGroup Condition="'$(InjectLockPolyfill)' == 'true' AND '$(_NeedsNet9Polyfills)' == 'true'">
-    <Compile Include="$(_SharedSourcePath)Polyfills/Lock.cs" LinkBase="Shared/Polyfills" InProject="false"/>
+    <Compile Include="$(_SharedSourcePath)MSBuild/Polyfills/Lock.cs" LinkBase="Shared/Polyfills" InProject="false"/>
   </ItemGroup>
 
   <!-- ─────────────────────────────────────────────────────────────────────────

tests/ANcpLua.Sdk.Tests/ClaudeBrainTests.cs

@@ -343,7 +343,7 @@ public void AddDirectoryBuildPropsFile(string postSdkContent, string preSdkConte
                        </Project>
                        """;
 
-        AddFile(RepositoryPaths.DirectoryBuildProps, content);
+        AddFile("Directory.Build.props", content);
     }
 
     /// <inheritdoc />