(ecs): create unit tests for drain hook lambda handler

### Describe the feature

We need to improve testing coverage of our Lambda handlers. The Lambda handler for [drain hooks](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/aws-ecs/lib/drain-hook/lambda-source/index.py) does not have corresponding unit tests. Unit tests should be created specifically for this handler.

### Use Case

Unit tests for the drain hook Lambda handler would improve test coverage and help us find bugs and prevent breaking changes.

### Proposed Solution

_No response_

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.103.1

### Environment details (OS name and version, etc.)

MacOS