Add unit tests for chunker module functions

[MarkEdmondson1234]

Expands the unit tests in tests/test_chunker.py to cover additional functions within the sunholo.chunker module, ensuring comprehensive testing and self-containment.

• Adds tests for send_doc_to_docstore: Mocks external calls and verifies the function's ability to process documents and return expected identifiers and processed documents.
• Introduces tests for create_big_doc: Validates the function's capability to merge document contents and metadata correctly, ensuring the creation of a 'big document' from multiple smaller ones.
• Implements tests for summarise_docs: Uses mocking to test the summarization logic, checking that summaries are correctly generated for given documents.
• Adds tests for upload_doc_images: Ensures the function correctly uploads images to a specified bucket and modifies the metadata accordingly, including the removal of the image_base64 key and setting the uploaded_to_bucket flag to true.

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:rocket:	This description was created by Ellipsis for commit e91bcfaec0169d2c7023a8aae33dc78e464a25c6

Summary:

Expands unit tests for the chunker module, enhancing test coverage for document processing and handling functions.

Key points:

• Adds unit tests in tests/test_chunker.py for send_doc_to_docstore, create_big_doc, summarise_docs, and upload_doc_images.
• Uses mocking to isolate functions and test functionality effectively.
• Ensures functions handle data correctly and modify metadata as expected.

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