Apache PDFBox - Merge PDFs

 Operation: mergePdfs

Input:
  pdfFiles: A List of InputStreams, where each InputStream is the binary content of a PDF file. Must contain at least two InputStreams.
  streamingHelper: MuleSoft StreamingHelper (for context/utilities - not directly used in logic shown).

Output:
  Result containing:
    - Merged PDF content (InputStream) as output.
    - PDF file attributes (PdfBoxFileAttributes) of the merged document as attributes.

Errors:
  PDF_PROCESSING_ERROR: If fewer than two PDF files are provided, or if the merge or saving fails.
  PDF_LOAD_FAILED: If the merged PDF document cannot be loaded for metadata extraction.
  PDF_METADATA_EXTRACTION_FAILED: If metadata cannot be retrieved from the merged document.
  IOException: If reading input streams or closing resources fails.

Steps:
1. Check the size of the input `pdfFiles` list.
2. If the size is less than 2, throw a ModuleException with PDF_PROCESSING_ERROR and a message indicating that at least two files are required.
3. Create a new ByteArrayOutputStream to write the merged PDF content to.
4. Create a new PDFMergerUtility instance.
5. Set the destination stream of the PDFMergerUtility to the ByteArrayOutputStream.
6. Initialize an empty List to store RandomAccessRead buffers created from the input streams.
7. Try Block:
   a. Iterate through each InputStream in the input `pdfFiles` list:
      i. Convert the current InputStream to a byte array using the `toByteArray` helper method.
      ii. Create a new RandomAccessReadBuffer from the byte array.
      iii. Add the created RandomAccessReadBuffer to the list of buffers.
      iv. Add the created RandomAccessReadBuffer as a source to the PDFMergerUtility.
   b. Call the `mergeDocuments(null)` method on the PDFMergerUtility to perform the merge operation.
   c. Get the byte array from the ByteArrayOutputStream (this is the merged PDF content).
   d. Try-with-Resources Block (for loading the merged document for metadata):
      i. Load the merged PDF byte array into a PDDocument using PDFBox Loader.
      ii. If loading fails, this block will throw an IOException, which will be caught by the outer catch block.
      iii. Extract metadata from the loaded merged PDDocument and get the size of the merged byte array using the `extractPdfMetadata` helper method.
      iv. Create a Result object containing:
          - A new InputStream created from the merged byte array as the output.
          - Set the media type to APPLICATION_OCTET_STREAM.
          - The extracted PdfBoxFileAttributes object as attributes.
      v. Return the Result object.
   e. End Try-with-Resources Block.
8. Catch Block (for IOException):
   a. If any IOException occurs during the Try Block (reading streams, merging, saving, loading merged doc), catch it.
   b. Throw a ModuleException with PDF_PROCESSING_ERROR and the original IOException as the cause.
9. Finally Block:
   a. Iterate through the list of created RandomAccessRead buffers.
   b. For each buffer, attempt to close it.
   c. If closing a buffer throws an IOException, log a warning but continue closing other buffers.