Add return type validation for MethodBodyGenerator (MSGH008)

Report MSGH008 diagnostic when a [MethodBodyGenerator] method's return type
is neither IMethodBodyGenerator (fluent pattern) nor the exact return type
of the target partial method (simple pattern). The diagnostic location
highlights only the return type keyword in the method declaration.

Co-authored-by: dex3r <3155725+dex3r@users.noreply.github.com>
Agent-Logs-Url: https://github.com/dex3r/EasySourceGenerators/sessions/8515330e-e7a2-438c-8d6a-d6c01baa8bbc

Author: Copilot

EasySourceGenerators.Generators/IncrementalGenerators/GeneratesMethodGenerationTargetCollector.cs

@@ -111,6 +111,18 @@ internal static List<GeneratesMethodGenerationTarget> Collect(
                 continue;
             }
 
+            string generatorReturnType = methodSymbol.ReturnType.ToDisplayString();
+            string targetReturnType = partialMethodSymbol.ReturnType.ToDisplayString();
+
+            if (generatorReturnType != IMethodImplementationGeneratorFullName && generatorReturnType != targetReturnType)
+            {
+                context.ReportDiagnostic(Diagnostic.Create(
+                    GeneratesMethodGeneratorDiagnostics.MethodBodyGeneratorInvalidReturnType,
+                    generatorMethod.ReturnType.GetLocation(),
+                    methodSymbol.Name));
+                continue;
+            }
+
             validMethods.Add(new GeneratesMethodGenerationTarget(
                 generatorMethod,
                 methodSymbol,