(aws-cloudwatch): Metric widget can't display composite alarm

### Describe the bug

This might actually be a bug in CloudWatch itself, but I didn't know where else to report it.

According to the [documentation](https://docs.aws.amazon.com/AmazonCloudWatch/latest/APIReference/CloudWatch-Dashboard-Body-Structure.html#CloudWatch-Dashboard-Properties-Metric-Widget-Object) of the CW metric widget body structure, it should be possible to use a metric widget to display an alarm, provided that the `annotations` property is used like so:

```json
"annotations": {
"alarms": [ "arn1" ]
}
```

As of today, in my CW dashboards, this only works for metric alarms, but not composite alarms. For a composite alarm, I get the message `Alarm FooCompositeAlarm in region is missing or deleted`. I don't have a good sense of when this stopped working, but it was working on June 9. Editing the dashboard source manually to change the widget type to `alarm`, using the same alarm ARN, results in a working widget that correctly reports the status of my composite alarm.

Things I've tried to isolate the root cause:
* Different browsers: Chrome and Firefox
* Incognito mode to exclude browser caching as a culprit

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Library Version

_No response_

### Expected Behavior

I expect my dashboard metric widget to display my composite alarm.

### Current Behavior

Metric widget claims the composite alarm doesn't exist, while I can clearly see it in CloudWatch, and I can graph it using an alarm widget.

### Reproduction Steps

Create a CloudWatch dashboard with a metric widget which displays a composite alarm.

The CDK code for generating the composite alarm from several individual metric alarms looks like:

```typescript
return new cw.CompositeAlarm(this.scope, `${feature.featureName}Alarm`, {
compositeAlarmName: generateName("AggregateAlarm"),
alarmRule: cw.AlarmRule.anyOf(...alarms),
alarmDescription: `Any alarm from the ${feature.featureName} subsystem has triggered.`,
});
```

Alarms then get added as metric widgets to the dashboard like so:

```typescript
const alarmWidgets = this.alarms.map(
(alarm) =>
new cloudwatch.AlarmWidget({
title: alarm.alarmName,
alarm: alarm,
}),
);

dashboard.addWidgets(new cloudwatch.Row(...alarmWidgets));
```

### Possible Solution

_No response_

### Additional Information/Context

_No response_

### AWS CDK Library version (aws-cdk-lib)

2.201.0

### AWS CDK CLI version

2.1003.0 (build b242c23)

### Node.js Version

v20.18.1

### OS

MacOS Sonoma 14.7.6

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at the CDK AlarmWidget entry point and reproduce the reported dashboard using a composite alarm, comparing the metric-widget and alarm-widget JSON it generates. Check whether the discrepancy is in CDK output or CloudWatch behavior; done means composite alarms display correctly or the issue is narrowed to an upstream CloudWatch defect.

Written by the indexing model from the issue text.