Unit Testing for Document Chunkers

[khaledsulayman]

Currently, the unit tests for the document chunkers primarily revolve around class instantiation, etc.

We should definitely add tests for the .chunk_documents() method in each chunker class. Additionally, it would be good to add tests around some of the custom logic we have for building chunks based on docling's .json outputs.

On the other hand, the DocumentChunker interface may change as a result of #334, so we should decide whether or not we want to hold off and do that first. Once that refactor is complete, we will only need to test one class and it should be easier for us to parametrize and test different filetypes.