refactor: extract type validation logic to the common library

CompositeKey.slnx

@@ -8,6 +8,7 @@
         <Project Path="src\CompositeKey\CompositeKey.csproj" Type="Classic C#" />
     </Folder>
     <Folder Name="/tests/">
+        <Project Path="src\CompositeKey.Analyzers.Common.UnitTests\CompositeKey.Analyzers.Common.UnitTests.csproj" Type="Classic C#" />
         <Project Path="src\CompositeKey.SourceGeneration.FunctionalTests\CompositeKey.SourceGeneration.FunctionalTests.csproj" Type="Classic C#" />
         <Project Path="src\CompositeKey.SourceGeneration.UnitTests\CompositeKey.SourceGeneration.UnitTests.csproj" Type="Classic C#" />
     </Folder>

src/CompositeKey.Analyzers.Common.UnitTests/CompositeKey.Analyzers.Common.UnitTests.csproj

@@ -0,0 +1,40 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+    <PropertyGroup>
+        <TargetFrameworks>net9.0;net8.0</TargetFrameworks>
+        <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
+
+        <IsPackable>false</IsPackable>
+        <IsTestProject>true</IsTestProject>
+    </PropertyGroup>
+
+    <ItemGroup>
+        <Using Include="Shouldly" />
+        <Using Include="Xunit" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="Microsoft.CodeAnalysis" />
+    </ItemGroup>
+
+    <ItemGroup>
+        <PackageReference Include="coverlet.collector">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+        <PackageReference Include="JUnitXml.TestLogger" />
+        <PackageReference Include="Microsoft.NET.Test.Sdk" />
+        <PackageReference Include="Shouldly" />
+        <PackageReference Include="xunit" />
+        <PackageReference Include="xunit.runner.visualstudio">
+            <PrivateAssets>all</PrivateAssets>
+            <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+        </PackageReference>
+    </ItemGroup>
+
+    <ItemGroup>
+        <ProjectReference Include="..\CompositeKey\CompositeKey.csproj" />
+        <ProjectReference Include="..\CompositeKey.Analyzers.Common\CompositeKey.Analyzers.Common.csproj" />
+    </ItemGroup>
+
+</Project>

src/CompositeKey.Analyzers.Common.UnitTests/TestHelpers/CompilationTestHelper.cs

@@ -0,0 +1,63 @@
+using System.Reflection;
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+
+namespace CompositeKey.Analyzers.Common.UnitTests.TestHelpers;
+
+/// <summary>
+/// Test helper for creating Roslyn compilation objects for testing validation logic.
+/// </summary>
+public static class CompilationTestHelper
+{
+    private static readonly Assembly SystemRuntimeAssembly = Assembly.Load(new AssemblyName("System.Runtime"));
+    private static readonly CSharpParseOptions DefaultParseOptions = new(LanguageVersion.CSharp11);
+
+    /// <summary>
+    /// Creates a C# compilation from source code.
+    /// </summary>
+    public static CSharpCompilation CreateCompilation(string source, string assemblyName = "TestAssembly")
+    {
+        List<MetadataReference> references =
+        [
+            MetadataReference.CreateFromFile(typeof(object).Assembly.Location),
+            MetadataReference.CreateFromFile(typeof(Guid).Assembly.Location),
+            MetadataReference.CreateFromFile(SystemRuntimeAssembly.Location),
+            MetadataReference.CreateFromFile(typeof(CompositeKeyAttribute).Assembly.Location),
+        ];
+
+        return CSharpCompilation.Create(
+            assemblyName,
+            [CSharpSyntaxTree.ParseText(source, DefaultParseOptions)],
+            references,
+            new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));
+    }
+
+    /// <summary>
+    /// Gets the semantic model and type declaration syntax for the first type in the compilation.
+    /// </summary>
+    public static (SemanticModel SemanticModel, TypeDeclarationSyntax TypeDeclaration, INamedTypeSymbol TypeSymbol)
+        GetFirstTypeInfo(CSharpCompilation compilation, string typeName)
+    {
+        var syntaxTree = compilation.SyntaxTrees.First();
+        var semanticModel = compilation.GetSemanticModel(syntaxTree);
+
+        var typeDeclaration = syntaxTree.GetRoot()
+            .DescendantNodes()
+            .OfType<TypeDeclarationSyntax>()
+            .First(t => t.Identifier.ValueText == typeName);
+
+        var typeSymbol = semanticModel.GetDeclaredSymbol(typeDeclaration)
+            ?? throw new InvalidOperationException($"Could not get type symbol for {typeName}");
+
+        return (semanticModel, typeDeclaration, typeSymbol);
+    }
+
+    /// <summary>
+    /// Gets the CompositeKeyConstructorAttribute type symbol from the compilation.
+    /// </summary>
+    public static INamedTypeSymbol? GetCompositeKeyConstructorAttributeSymbol(CSharpCompilation compilation)
+    {
+        return compilation.GetTypeByMetadataName("CompositeKey.CompositeKeyConstructorAttribute");
+    }
+}