Composite alarm creation fails when child alarms don't exist

Creating a composite alarm that references non-existent metric alarms in its AlarmRule triggers a ValidationError during PutCompositeAlarm operation, preventing alarm creation.

Ensure all metric alarms referenced in the composite alarm rule exist before creating the composite alarm. Use DescribeAlarms to verify child alarms are present. If creating alarms programmatically, create metric alarms before composite alarms that reference them.