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

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

Start in lambda-handler/get-document-handler and read the handler flow to identify the DynamoDB methods it calls. Use the linked mocking guide as reference for isolating those methods, then add unit-test coverage for the handler and verify the tests pass.

Written by the indexing model from the issue text.