fix(stepfunctions): the retry field in CustomState is not iterable (#29403)

### Issue # (if applicable)

Closes #29274 

### Reason for this change

CDK users were unable to specify their retry strategy if it was specified inline in their ASL state machine definition

### Description of changes

Checks if the state definition has an inline retry policy defined. If it does, add it to the existing strategy defined using `addRetry` (if there is one defined, this is where it was failing before)

### Description of how you validated changes

Added unit and integration tests

### Checklist
- [x] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)

----

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: abdelnn

packages/@aws-cdk-testing/framework-integ/test/aws-stepfunctions/test/integ.custom-state.js.snapshot/aws-stepfunctions-custom-state-integ.assets.json

@@ -20,7 +20,7 @@
   "StateMachine2E01A3A5": {
    "Type": "AWS::StepFunctions::StateMachine",
    "Properties": {
-    "DefinitionString": "{\"StartAt\":\"my custom task\",\"States\":{\"my custom task\":{\"Next\":\"final step\",\"Type\":\"Task\",\"Resource\":\"arn:aws:states:::dynamodb:putItem\",\"Parameters\":{\"TableName\":\"my-cool-table\",\"Item\":{\"id\":{\"S\":\"my-entry\"}}},\"ResultPath\":null,\"Retry\":[{\"ErrorEquals\":[\"States.Timeout\"],\"IntervalSeconds\":10,\"MaxAttempts\":5},{\"ErrorEquals\":[\"States.Permissions\"],\"IntervalSeconds\":20,\"MaxAttempts\":2}],\"Catch\":[{\"ErrorEquals\":[\"States.ALL\"],\"Next\":\"failed\"}]},\"final step\":{\"Type\":\"Pass\",\"End\":true},\"failed\":{\"Type\":\"Fail\",\"Error\":\"DidNotWork\",\"Cause\":\"We got stuck\"}},\"TimeoutSeconds\":30}",
+    "DefinitionString": "{\"StartAt\":\"my custom task\",\"States\":{\"my custom task\":{\"Next\":\"my custom task with inline Retriers\",\"Type\":\"Task\",\"Resource\":\"arn:aws:states:::dynamodb:putItem\",\"Parameters\":{\"TableName\":\"my-cool-table\",\"Item\":{\"id\":{\"S\":\"my-entry\"}}},\"ResultPath\":null,\"Retry\":[{\"ErrorEquals\":[\"States.Timeout\"],\"IntervalSeconds\":10,\"MaxAttempts\":5},{\"ErrorEquals\":[\"States.Permissions\"],\"IntervalSeconds\":20,\"MaxAttempts\":2}],\"Catch\":[{\"ErrorEquals\":[\"States.ALL\"],\"Next\":\"failed\"}]},\"my custom task with inline Retriers\":{\"Next\":\"final step\",\"Type\":\"Task\",\"Resource\":\"arn:aws:states:::dynamodb:putItem\",\"Parameters\":{\"TableName\":\"my-cool-table\",\"Item\":{\"id\":{\"S\":\"my-entry\"}}},\"ResultPath\":null,\"Retry\":[{\"ErrorEquals\":[\"States.Permissions\"],\"IntervalSeconds\":20,\"MaxAttempts\":2}]},\"final step\":{\"Type\":\"Pass\",\"End\":true},\"failed\":{\"Type\":\"Fail\",\"Error\":\"DidNotWork\",\"Cause\":\"We got stuck\"}},\"TimeoutSeconds\":30}",
     "RoleArn": {
      "Fn::GetAtt": [
       "StateMachineRoleB840431D",

packages/@aws-cdk-testing/framework-integ/test/aws-stepfunctions/test/integ.custom-state.js.snapshot/aws-stepfunctions-custom-state-integ.template.json

@@ -39,14 +39,18 @@ const custom = new sfn.CustomState(stack, 'my custom task', {
   stateJson,
 });
 
+const customWithInlineRetry = new sfn.CustomState(stack, 'my custom task with inline Retriers', {
+  stateJson,
+});
+
 custom.addCatch(failure);
 custom.addRetry({
   errors: [sfn.Errors.TIMEOUT],
   interval: cdk.Duration.seconds(10),
   maxAttempts: 5,
 });
 
-const chain = sfn.Chain.start(custom).next(finalStatus);
+const chain = sfn.Chain.start(custom).next(customWithInlineRetry).next(finalStatus);
 
 const sm = new sfn.StateMachine(stack, 'StateMachine', {
   definition: chain,

packages/@aws-cdk-testing/framework-integ/test/aws-stepfunctions/test/integ.custom-state.js.snapshot/manifest.json

@@ -74,9 +74,9 @@ export class CustomState extends State implements IChainable, INextable {
       ...this.renderRetryCatch(),
     };
 
-    // merge the Retry filed defined in the stateJson into the state
+    // merge the Retry field defined in the stateJson into the state
     if (Array.isArray(this.stateJson.Retry)) {
-      state.Retry = [...state.Retry, ...this.stateJson.Retry];
+      state.Retry = Array.isArray(state.Retry) ? [...state.Retry, ...this.stateJson.Retry] : [...this.stateJson.Retry];
     }
 
     return state;

packages/@aws-cdk-testing/framework-integ/test/aws-stepfunctions/test/integ.custom-state.js.snapshot/tree.json

@@ -242,4 +242,71 @@ describe('Custom State', () => {
       },
     );
   });
+
+  test('expect retry to not fail when specifying strategy inline', () => {
+    // GIVEN
+    const custom = new sfn.CustomState(stack, 'Custom', {
+      stateJson: {
+        Type: 'Task',
+        Resource: 'arn:aws:states:::dynamodb:putItem',
+        Parameters: {
+          TableName: 'MyTable',
+          Item: {
+            id: {
+              S: 'MyEntry',
+            },
+          },
+        },
+        ResultPath: null,
+        Retry: [
+          {
+            ErrorEquals: [
+              'Lambda.ServiceException',
+              'Lambda.AWSLambdaException',
+              'Lambda.SdkClientException',
+              'Lambda.TooManyRequestsException',
+            ],
+            IntervalSeconds: 20,
+            MaxAttempts: 2,
+          },
+        ],
+      },
+    });
+    const chain = sfn.Chain.start(custom);
+
+    // THEN
+    expect(render(stack, chain)).toStrictEqual(
+      {
+        StartAt: 'Custom',
+        States: {
+          Custom: {
+            Type: 'Task',
+            Resource: 'arn:aws:states:::dynamodb:putItem',
+            Parameters: {
+              TableName: 'MyTable',
+              Item: {
+                id: {
+                  S: 'MyEntry',
+                },
+              },
+            },
+            ResultPath: null,
+            Retry: [
+              {
+                ErrorEquals: [
+                  'Lambda.ServiceException',
+                  'Lambda.AWSLambdaException',
+                  'Lambda.SdkClientException',
+                  'Lambda.TooManyRequestsException',
+                ],
+                IntervalSeconds: 20,
+                MaxAttempts: 2,
+              },
+            ],
+            End: true,
+          },
+        },
+      },
+    );
+  });
 });