fix: replace invalid MultiValueHeadersEnabled with TargetGroupAttributes for ALB TargetGroup

MultiValueHeadersEnabled is not a valid CloudFormation property on
AWS::ElasticLoadBalancingV2::TargetGroup. Multi-value headers must be
configured via TargetGroupAttributes with the key
lambda.multi_value_headers.enabled.

Changes:
- CloudFormationWriter: emit TargetGroupAttributes array when
  MultiValueHeaders=true, remove it when false (instead of setting
  the non-existent MultiValueHeadersEnabled property)
- albEvents.template snapshot: remove MultiValueHeadersEnabled=false,
  add TargetGroupAttributes for the multi-value-headers case
- ALBEventsTests: update assertions to check TargetGroupAttributes
  existence instead of MultiValueHeadersEnabled
- TestServerlessApp.ALB/serverless.template: remove stale
  MultiValueHeadersEnabled from both target groups

Author: GarrettBeatty

Libraries/src/Amazon.Lambda.Annotations.SourceGenerator/Writers/CloudFormationWriter.cs

@@ -633,7 +633,26 @@ private void ProcessAlbApiAttribute(ILambdaFunctionSerializable lambdaFunction,
                 _templateWriter.SetToken($"{tgPath}.Metadata.Tool", CREATION_TOOL);
                 _templateWriter.SetToken($"{tgPath}.DependsOn", permissionName);
                 _templateWriter.SetToken($"{tgPath}.Properties.TargetType", "lambda");
-                _templateWriter.SetToken($"{tgPath}.Properties.MultiValueHeadersEnabled", att.MultiValueHeaders);
+
+                // MultiValueHeaders must be set via TargetGroupAttributes, not as a top-level property.
+                // The CFN property "MultiValueHeadersEnabled" does not exist on AWS::ElasticLoadBalancingV2::TargetGroup.
+                if (att.MultiValueHeaders)
+                {
+                    _templateWriter.SetToken($"{tgPath}.Properties.TargetGroupAttributes",
+                        new List<Dictionary<string, string>>
+                        {
+                            new Dictionary<string, string>
+                            {
+                                { "Key", "lambda.multi_value_headers.enabled" },
+                                { "Value", "true" }
+                            }
+                        }, TokenType.List);
+                }
+                else
+                {
+                    _templateWriter.RemoveToken($"{tgPath}.Properties.TargetGroupAttributes");
+                }
+
                 _templateWriter.SetToken($"{tgPath}.Properties.Targets", new List<Dictionary<string, object>>
                 {
                     new Dictionary<string, object>

Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/Snapshots/ServerlessTemplates/albEvents.template

@@ -44,7 +44,6 @@
       "DependsOn": "ALBHelloWorldALBPermission",
       "Properties": {
         "TargetType": "lambda",
-        "MultiValueHeadersEnabled": false,
         "Targets": [
           {
             "Id": {
@@ -124,7 +123,12 @@
       "DependsOn": "ALBWithOptionsALBPermission",
       "Properties": {
         "TargetType": "lambda",
-        "MultiValueHeadersEnabled": true,
+        "TargetGroupAttributes": [
+          {
+            "Key": "lambda.multi_value_headers.enabled",
+            "Value": "true"
+          }
+        ],
         "Targets": [
           {
             "Id": {

Libraries/test/Amazon.Lambda.Annotations.SourceGenerators.Tests/WriterTests/ALBEventsTests.cs

@@ -56,7 +56,8 @@ public void ALBApiAttribute_GeneratesTargetGroupListenerRuleAndPermission(CloudF
             Assert.Equal("Amazon.Lambda.Annotations", templateWriter.GetToken<string>("Resources.HelloWorldALBTargetGroup.Metadata.Tool"));
             Assert.Equal("HelloWorldALBPermission", templateWriter.GetToken<string>("Resources.HelloWorldALBTargetGroup.DependsOn"));
             Assert.Equal("lambda", templateWriter.GetToken<string>("Resources.HelloWorldALBTargetGroup.Properties.TargetType"));
-            Assert.False(templateWriter.GetToken<bool>("Resources.HelloWorldALBTargetGroup.Properties.MultiValueHeadersEnabled"));
+            // When MultiValueHeaders is false (default), no TargetGroupAttributes should be present
+            Assert.False(templateWriter.Exists("Resources.HelloWorldALBTargetGroup.Properties.TargetGroupAttributes"));
 
             // Verify ListenerRule resource
             Assert.True(templateWriter.Exists("Resources.HelloWorldALBListenerRule"));
@@ -99,7 +100,7 @@ public void ALBApiAttribute_WithTemplateReference_UsesRef(CloudFormationTemplate
         [Theory]
         [InlineData(CloudFormationTemplateFormat.Json)]
         [InlineData(CloudFormationTemplateFormat.Yaml)]
-        public void ALBApiAttribute_WithMultiValueHeaders_SetsEnabled(CloudFormationTemplateFormat templateFormat)
+        public void ALBApiAttribute_WithMultiValueHeaders_SetsTargetGroupAttributes(CloudFormationTemplateFormat templateFormat)
         {
             // ARRANGE
             var mockFileManager = GetMockFileManager(string.Empty);
@@ -122,7 +123,8 @@ public void ALBApiAttribute_WithMultiValueHeaders_SetsEnabled(CloudFormationTemp
 
             // ASSERT
             templateWriter.Parse(mockFileManager.ReadAllText(ServerlessTemplateFilePath));
-            Assert.True(templateWriter.GetToken<bool>("Resources.MyFunctionALBTargetGroup.Properties.MultiValueHeadersEnabled"));
+            // When MultiValueHeaders is true, TargetGroupAttributes should contain the lambda.multi_value_headers.enabled attribute
+            Assert.True(templateWriter.Exists("Resources.MyFunctionALBTargetGroup.Properties.TargetGroupAttributes"));
         }
 
         [Theory]

Libraries/test/TestServerlessApp.ALB/serverless.template

@@ -208,7 +208,6 @@
       "DependsOn": "ALBHelloALBPermission",
       "Properties": {
         "TargetType": "lambda",
-        "MultiValueHeadersEnabled": false,
         "Targets": [
           {
             "Id": {
@@ -227,9 +226,6 @@
         "Tool": "Amazon.Lambda.Annotations"
       },
       "Properties": {
-        "ListenerArn": {
-          "Ref": "ALBTestListener"
-        },
         "Priority": 1,
         "Conditions": [
           {
@@ -246,7 +242,10 @@
               "Ref": "ALBHelloALBTargetGroup"
             }
           }
-        ]
+        ],
+        "ListenerArn": {
+          "Ref": "ALBTestListener"
+        }
       }
     },
     "ALBHealth": {
@@ -290,7 +289,6 @@
       "DependsOn": "ALBHealthALBPermission",
       "Properties": {
         "TargetType": "lambda",
-        "MultiValueHeadersEnabled": false,
         "Targets": [
           {
             "Id": {
@@ -309,9 +307,6 @@
         "Tool": "Amazon.Lambda.Annotations"
       },
       "Properties": {
-        "ListenerArn": {
-          "Ref": "ALBTestListener"
-        },
         "Priority": 2,
         "Conditions": [
           {
@@ -328,7 +323,10 @@
               "Ref": "ALBHealthALBTargetGroup"
             }
           }
-        ]
+        ],
+        "ListenerArn": {
+          "Ref": "ALBTestListener"
+        }
       }
     }
   },