feat(lambda-get-document-tree-handler): add Unit test

Add a Unit test for [get-document-tree-handler](https://github.com/fynnfluegge/rocketnotes/tree/main/lambda-handler/get-document-tree-handler). See this [guide](https://adityarama1210.medium.com/golang-mocking-a-function-for-unit-testing-497b43ad3409) how to mock DynamoDB methods.

Open the lambda-handler/get-document-tree-handler directory and review the get-document-tree-handler implementation. Use the linked guide as a reference for mocking DynamoDB methods. Done means the handler has a unit test covering its DynamoDB interactions and the test passes.

Written by the indexing model from the issue text.