fix: address second round of PR review feedback

- Add file-count assertion to snapshot test for 5118
- Replace silent typeof(object) fallback with defensive throw
- Add comment explaining why Factory makes ClassProvidingDataSource unnecessary

TUnit.Core.SourceGenerator.Tests/Bugs/5118/Tests5118.cs

@@ -11,5 +11,6 @@ public Task Test() => RunTest(Path.Combine(Git.RootDirectory.FullName,
             "AsyncClassMethodDataSourceTests.cs"),
         async generatedFiles =>
         {
-            });
+            await Assert.That(generatedFiles).HasCount().EqualTo(1);
+        });
 }

TUnit.Core.SourceGenerator/Generators/TestMetadataGenerator.cs

@@ -903,7 +903,9 @@ private static void GenerateMethodDataSourceAttribute(CodeWriter writer, Attribu
         }
         else
         {
-            // MethodDataSource(string) constructor
+            // MethodDataSource(string) constructor — ClassProvidingDataSource is intentionally null here
+            // because the Factory delegate (set below) already encodes the correct target type directly.
+            // The runtime attribute only needs ClassProvidingDataSource for reflection-based lookup.
             writer.AppendLine($"new {attrTypeName}(\"{methodName}\")");
         }
         writer.AppendLine("{");

TUnit.Core/Attributes/TestData/MethodDataSourceAttribute.cs

@@ -326,7 +326,8 @@ public MethodDataSourceAttribute(
             {
                 ParameterMetadata param => param.Type,
                 PropertyMetadata prop => prop.Type,
-                _ => typeof(object)
+                _ => throw new InvalidOperationException(
+                    $"Unexpected member metadata type {members[i].GetType().Name} in GetMemberTypes")
             };
         }
         return types;