Fix return type diagnostic conditions (#30)

Author: Dreamescaper

ServiceScan.SourceGenerator/DependencyInjectionGenerator.ParseMethodModel.cs

@@ -84,7 +84,7 @@ public partial class DependencyInjectionGenerator
         }
         else
         {
-            if (method.IsExtensionMethod && !method.ReturnsVoid &&
+            if (!method.ReturnsVoid &&
                 (method.Parameters.Length == 0 || !SymbolEqualityComparer.Default.Equals(method.Parameters[0].Type, method.ReturnType)))
             {
                 return Diagnostic.Create(WrongReturnTypeForCustomHandler, method.Locations[0]);

ServiceScan.SourceGenerator/DiagnosticDescriptors.cs

@@ -55,7 +55,7 @@ public static class DiagnosticDescriptors
 
     public static readonly DiagnosticDescriptor WrongReturnTypeForCustomHandler = new("DI0009",
         "Wrong return type",
-        "Method with CustomHandler must return void or 'this' parameter type",
+        "Method with CustomHandler must return void or the type of its first parameter",
         "Usage",
         DiagnosticSeverity.Error,
         true);