fix(should): inject public OverloadResolutionPriorityAttribute polyfi…

…ll in net8 generator tests

Net8.0 BCL lacks OverloadResolutionPriorityAttribute. The Polyfill copy
compiled into the test assembly is internal, so the synthetic GeneratorTest
compilation can't bind constructor arguments — generator returns priority 0
and snapshot snapshot diverges from the verified output that includes
[OverloadResolutionPriority(N)]. Inject a public copy into the compilation
under #if NET8_0 so attribute resolves consistently across all multi-target
TFMs.

TUnit.Assertions.Should.SourceGenerator.Tests/ShouldExtensionGeneratorTests.cs

@@ -329,9 +329,26 @@ public static SetAssertion<TItem> That<TItem>(
     /// </summary>
     private static async Task<string> RunGenerator(string userSource, [CallerMemberName] string testName = "")
     {
+        // On net8.0 hosts, OverloadResolutionPriorityAttribute is missing from the BCL and the
+        // Polyfill copy compiled into this test assembly is internal — invisible to the synthetic
+        // GeneratorTest compilation. Inject a public copy so the attribute resolves consistently
+        // across all TFMs the test multi-targets.
+        var inputTrees = new List<SyntaxTree> { CSharpSyntaxTree.ParseText(userSource) };
+#if NET8_0
+        inputTrees.Add(CSharpSyntaxTree.ParseText("""
+            namespace System.Runtime.CompilerServices;
+            [System.AttributeUsage(System.AttributeTargets.Method | System.AttributeTargets.Constructor | System.AttributeTargets.Property, Inherited = false)]
+            public sealed class OverloadResolutionPriorityAttribute : System.Attribute
+            {
+                public OverloadResolutionPriorityAttribute(int priority) => Priority = priority;
+                public int Priority { get; }
+            }
+            """));
+#endif
+
         var compilation = CSharpCompilation.Create(
             assemblyName: "GeneratorTest",
-            syntaxTrees: [CSharpSyntaxTree.ParseText(userSource)],
+            syntaxTrees: inputTrees,
             references: GetReferences(),
             options: new CSharpCompilationOptions(OutputKind.DynamicallyLinkedLibrary));
 
@@ -342,11 +359,11 @@ private static async Task<string> RunGenerator(string userSource, [CallerMemberN
         await Assert.That(diagnostics.Length).IsEqualTo(0)
             .Because("Generator should not emit diagnostics for valid input");
 
-        var trees = updatedCompilation.SyntaxTrees
-            .Where(t => t != compilation.SyntaxTrees[0])
+        var generatedTrees = updatedCompilation.SyntaxTrees
+            .Where(t => !compilation.SyntaxTrees.Contains(t))
             .Select(t => t.ToString());
 
-        var combined = string.Join("\n//------\n", trees);
+        var combined = string.Join("\n//------\n", generatedTrees);
 
         await Verify(combined)
             .UseFileName($"{nameof(ShouldExtensionGeneratorTests)}.{testName}")