Add option to skip corrupt PDFs in PDFMergerUtility with improved exception handling

[SwethaMuthuvel]

What This PR Does

This pull request improves the robustness and debuggability of PDFMergerUtility by:

1. Adding a skipCorruptFiles flag

   • Allows users to skip unreadable or corrupt PDF files during merge.
   • Default behavior remains unchanged (i.e., throws on error).

2. Wrapping IOException with source context

   • Converts vague errors like:

     IOException: Could not parse object stream
     

     into more useful messages like:

     IOException: Failed to load PDF from source: /path/to/file.pdf
     

   • Helps identify exactly which file failed.

3. Applied consistently in both merge modes

   • optimizedMergeDocuments(...)
   • legacyMergeDocuments(...)
   • Added warning logs when skipping files.

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Why This Helps

• Improves debuggability — pinpoints which file caused the failure.
• Makes batch operations resilient — avoids total failure from one bad input.
• Scales better — suitable for bulk merging scenarios.
• Does not break existing behavior — opt-in via setSkipCorruptFiles(true).