fix(autoscaling): step scaling without adjustment type fails (#29158)

### Reason for this change

Step Scaling without `adjustmentType` fails with CFn error `You must specify an AdjustmentType for policy type: StepScaling`.

- Reproduction code
```ts
const asg = new autoscaling.AutoScalingGroup(stack, 'ASG', {
  vpc,
  instanceType: ec2.InstanceType.of(ec2.InstanceClass.BURSTABLE2, ec2.InstanceSize.MICRO),
  machineImage: new ec2.AmazonLinuxImage({ generation: ec2.AmazonLinuxGeneration.AMAZON_LINUX_2 }),
});

asg.scaleOnMetric('StepScaling', {
  metric: new cloudwatch.Metric({ namespace: 'AWS/EC2', metricName: 'CPUUtilization', dimensionsMap: { AutoScalingGroupName: asg.autoScalingGroupName } }),
  scalingSteps: [
    { upper: 10, change: -1 },
    { lower: 50, change: +1 },
    { lower: 90, change: +2 },
  ],
  evaluationPeriods: 10,
  datapointsToAlarm: 5,
  metricAggregationType: autoscaling.MetricAggregationType.MAXIMUM,

  // adjustmentType: autoscaling.AdjustmentType.CHANGE_IN_CAPACITY,
});
```

### Description of changes

According to [the CDK code](https://github.com/aws/aws-cdk/blob/v2.122.0/packages/aws-cdk-lib/aws-autoscaling/lib/step-scaling-policy.ts#L105-L115), `CHANGE_IN_CAPACITY` will be used if `adjustmentType` is not specified in the prop. But the variable is not passed into `StepScalingAction` construct (instead `props.adjustmentType` is passed as is).

[The documentation](https://github.com/aws/aws-cdk/blob/v2.122.0/packages/aws-cdk-lib/aws-autoscaling/lib/step-scaling-policy.ts#L26) also says that the default value is `CHANGE_IN_CAPACITY`.

So we should use `adjustmentType` instead of `props.adjustmentType`.

```ts
    const adjustmentType = props.adjustmentType || AdjustmentType.CHANGE_IN_CAPACITY;
    const changesAreAbsolute = adjustmentType === AdjustmentType.EXACT_CAPACITY;
    // ...
    // ...

      this.lowerAction = new StepScalingAction(this, 'LowerPolicy', {
        // adjustmentType: props.adjustmentType,
        adjustmentType,
```

**The fact that the error occurs if `props.adjustmentType` is not specified means that the user's successful existing stack always specifies `props.adjustmentType`. In other words, no change to the existing resource will occur due to this change, so there is no need for a feature flag.**

### Description of how you validated changes

~~Unit tests without integ tests, because this PR could cover this bug by unit tests.~~
Both unit and integ 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*

packages/@aws-cdk-testing/framework-integ/test/aws-autoscaling/test/integ.asg-step-scaling.js.snapshot/autoscaling-step-scaling.template.json

@@ -643,6 +643,99 @@
     "Statistic": "Average",
     "Threshold": 50
    }
+  },
+  "ASGStepScalingWithDefaultAdjustmentTypeLowerPolicyA6C1EA33": {
+   "Type": "AWS::AutoScaling::ScalingPolicy",
+   "Properties": {
+    "AdjustmentType": "ChangeInCapacity",
+    "AutoScalingGroupName": {
+     "Ref": "ASG46ED3070"
+    },
+    "MetricAggregationType": "Maximum",
+    "PolicyType": "StepScaling",
+    "StepAdjustments": [
+     {
+      "MetricIntervalUpperBound": 0,
+      "ScalingAdjustment": -1
+     }
+    ]
+   }
+  },
+  "ASGStepScalingWithDefaultAdjustmentTypeLowerAlarmF9F52487": {
+   "Type": "AWS::CloudWatch::Alarm",
+   "Properties": {
+    "AlarmActions": [
+     {
+      "Ref": "ASGStepScalingWithDefaultAdjustmentTypeLowerPolicyA6C1EA33"
+     }
+    ],
+    "AlarmDescription": "Lower threshold scaling alarm",
+    "ComparisonOperator": "LessThanOrEqualToThreshold",
+    "DatapointsToAlarm": 5,
+    "Dimensions": [
+     {
+      "Name": "AutoScalingGroupName",
+      "Value": {
+       "Ref": "ASG46ED3070"
+      }
+     }
+    ],
+    "EvaluationPeriods": 10,
+    "MetricName": "DiskWriteOps",
+    "Namespace": "AWS/EC2",
+    "Period": 300,
+    "Statistic": "Average",
+    "Threshold": 100
+   }
+  },
+  "ASGStepScalingWithDefaultAdjustmentTypeUpperPolicy08CC2D99": {
+   "Type": "AWS::AutoScaling::ScalingPolicy",
+   "Properties": {
+    "AdjustmentType": "ChangeInCapacity",
+    "AutoScalingGroupName": {
+     "Ref": "ASG46ED3070"
+    },
+    "MetricAggregationType": "Maximum",
+    "PolicyType": "StepScaling",
+    "StepAdjustments": [
+     {
+      "MetricIntervalLowerBound": 0,
+      "MetricIntervalUpperBound": 200,
+      "ScalingAdjustment": 1
+     },
+     {
+      "MetricIntervalLowerBound": 200,
+      "ScalingAdjustment": 2
+     }
+    ]
+   }
+  },
+  "ASGStepScalingWithDefaultAdjustmentTypeUpperAlarm2379E17B": {
+   "Type": "AWS::CloudWatch::Alarm",
+   "Properties": {
+    "AlarmActions": [
+     {
+      "Ref": "ASGStepScalingWithDefaultAdjustmentTypeUpperPolicy08CC2D99"
+     }
+    ],
+    "AlarmDescription": "Upper threshold scaling alarm",
+    "ComparisonOperator": "GreaterThanOrEqualToThreshold",
+    "DatapointsToAlarm": 5,
+    "Dimensions": [
+     {
+      "Name": "AutoScalingGroupName",
+      "Value": {
+       "Ref": "ASG46ED3070"
+      }
+     }
+    ],
+    "EvaluationPeriods": 10,
+    "MetricName": "DiskWriteOps",
+    "Namespace": "AWS/EC2",
+    "Period": 300,
+    "Statistic": "Average",
+    "Threshold": 300
+   }
   }
  },
  "Parameters": {

packages/@aws-cdk-testing/framework-integ/test/aws-autoscaling/test/integ.asg-step-scaling.ts

@@ -30,6 +30,17 @@ asg.scaleOnMetric('StepScaling', {
   datapointsToAlarm: 5,
   metricAggregationType: autoscaling.MetricAggregationType.MAXIMUM,
 });
+asg.scaleOnMetric('StepScalingWithDefaultAdjustmentType', {
+  metric: new cloudwatch.Metric({ namespace: 'AWS/EC2', metricName: 'DiskWriteOps', dimensionsMap: { AutoScalingGroupName: asg.autoScalingGroupName } }),
+  scalingSteps: [
+    { upper: 100, change: -1 },
+    { lower: 300, change: +1 },
+    { lower: 500, change: +2 },
+  ],
+  evaluationPeriods: 10,
+  datapointsToAlarm: 5,
+  metricAggregationType: autoscaling.MetricAggregationType.MAXIMUM,
+});
 
 new integ.IntegTest(app, 'autoscaling-step-scaling-integ', {
   testCases: [stack],

packages/aws-cdk-lib/aws-autoscaling/lib/step-scaling-policy.ts

@@ -147,7 +147,7 @@ export class StepScalingPolicy extends Construct {
       const threshold = intervals[alarms.lowerAlarmIntervalIndex].upper;
 
       this.lowerAction = new StepScalingAction(this, 'LowerPolicy', {
-        adjustmentType: props.adjustmentType,
+        adjustmentType,
         cooldown: props.cooldown,
         estimatedInstanceWarmup: props.estimatedInstanceWarmup,
         metricAggregationType: props.metricAggregationType ?? aggregationTypeFromMetric(props.metric),
@@ -179,7 +179,7 @@ export class StepScalingPolicy extends Construct {
       const threshold = intervals[alarms.upperAlarmIntervalIndex].lower;
 
       this.upperAction = new StepScalingAction(this, 'UpperPolicy', {
-        adjustmentType: props.adjustmentType,
+        adjustmentType,
         cooldown: props.cooldown,
         estimatedInstanceWarmup: props.estimatedInstanceWarmup,
         metricAggregationType: props.metricAggregationType ?? aggregationTypeFromMetric(props.metric),