Fix build issues introduced by recent package upgrades

- Add Microsoft.Build.Framework 18.4.0 with ExcludeAssets=runtime/PrivateAssets=all
  to Directory.Packages.props and Integration.Tests project to resolve MSBL001 error
  introduced by Microsoft.Build.Locator 1.11.2

- Change GuidelineXmlToMD.Test to target net10.0 only (matching the tool project's
  TargetFramework) to fix NU1201 TFM compatibility restore errors

- Fix pre-existing nullable reference errors now surfaced in Tools build:
  - GuidelineXmlFileReader.cs: null-check Root before iterating, use ?? for attributes
  - MdExtensions.cs: remove unnecessary ?. on already-validated str parameter
  - MdText.cs: use ?? string.Empty for ToString() nullable return
  - GuidelineXmlFileReaderTest.cs: add ! null-forgiving on Directory.GetParent chain

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: BenjaminMichaelis

Directory.Packages.props

@@ -6,6 +6,7 @@
   <ItemGroup>
     <PackageVersion Include="coverlet.collector" Version="8.0.0" />
     <PackageVersion Include="IntelliTect.Analyzers" Version="0.2.0" />
+    <PackageVersion Include="Microsoft.Build.Framework" Version="18.4.0" />
     <PackageVersion Include="Microsoft.Build.Locator" Version="1.11.2" />
     <PackageVersion Include="Microsoft.CodeAnalysis.Analyzers" Version="5.3.0" />
     <PackageVersion Include="Microsoft.CodeAnalysis.CSharp" Version="5.3.0" />

IntelliTect.Analyzer/IntelliTect.Analyzer.Integration.Tests/IntelliTect.Analyzer.Integration.Tests.csproj

@@ -14,6 +14,7 @@
       <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
     </PackageReference>
 
+    <PackageReference Include="Microsoft.Build.Framework" ExcludeAssets="runtime" PrivateAssets="all" />
     <PackageReference Include="Microsoft.Build.Locator" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp.Workspaces" />

Tools/XMLtoMD/GuidelineXmlToMD.Test/GuidelineXmlFileReaderTest.cs

@@ -13,7 +13,7 @@ public class GuidelineXmlFileReaderTest
         public void ReadExisitingGuidelinesFile_ExistingFile_ReadsGuidlines()
         {
             // Arrange
-            var projectPath = Directory.GetParent(Environment.CurrentDirectory).Parent.Parent.FullName;
+            var projectPath = Directory.GetParent(Environment.CurrentDirectory)!.Parent!.Parent!.FullName;
             var testPath = Path.Combine(projectPath, @"Data\", "TestGuidelines.xml");
 
             // Act

Tools/XMLtoMD/GuidelineXmlToMD.Test/GuidelineXmlToMD.Test.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net8.0;net9.0;net10.0</TargetFrameworks>
+    <TargetFramework>net10.0</TargetFramework>
     <NoWarn>CA2007,CA1815,CA1303,CA1707,CA1305,IDE0008,INTL0003</NoWarn>
     <IsPackable>false</IsPackable>
   </PropertyGroup>

Tools/XMLtoMD/GuidelineXmlToMD/GuidelineXmlFileReader.cs

@@ -19,14 +19,17 @@ public static ICollection<Guideline> ReadExisitingGuidelinesFile(string pathToEx
 
             HashSet<Guideline> guidelines = [];
 
+            if (previousGuidelines.Root is null)
+                return guidelines;
+
             foreach (XElement guidelineFromXml in previousGuidelines.Root.DescendantNodes().OfType<XElement>())
             {
                 Guideline guideline = new Guideline(
-                    Key: guidelineFromXml.Attribute(_Key)?.Value,
-                    Text: guidelineFromXml?.Value,
-                    Severity: guidelineFromXml.Attribute(_Severity)?.Value,
-                    Section: guidelineFromXml.Attribute(_Section)?.Value,
-                    Subsection: guidelineFromXml.Attribute(_Subsection)?.Value
+                    Key: guidelineFromXml.Attribute(_Key)?.Value ?? string.Empty,
+                    Text: guidelineFromXml.Value,
+                    Severity: guidelineFromXml.Attribute(_Severity)?.Value ?? string.Empty,
+                    Section: guidelineFromXml.Attribute(_Section)?.Value ?? string.Empty,
+                    Subsection: guidelineFromXml.Attribute(_Subsection)?.Value ?? string.Empty
                 );
                 guidelines.Add(guideline);
             }

Tools/XMLtoMD/GuidelineXmlToMD/MarkdownOut/MdExtensions.cs

@@ -34,7 +34,7 @@ public static string StyleSubstring(this string str, string substring, MdStyle s
             }
 
             if (!firstOnly) {
-                return str?.Replace(substring, MdText.Style(substring, style), System.StringComparison.Ordinal);
+                return str.Replace(substring, MdText.Style(substring, style), System.StringComparison.Ordinal);
             }
             int pos = str.IndexOf(substring, System.StringComparison.Ordinal);
             if (pos < 0) {

Tools/XMLtoMD/GuidelineXmlToMD/MarkdownOut/MdText.cs

@@ -159,7 +159,7 @@ public static string Format(object text, MdFormat format) {
 
         public static string RemoveInvalidMDLinkCharacters(object text)
         {
-            return Regex.Replace(text.ToString(), "[^A-Za-z]", "");
+            return Regex.Replace(text.ToString() ?? string.Empty, "[^A-Za-z]", "");
         }
 
         /// <summary>