Fix handling of signature fields with multiple instances

[WitkacDevelopers]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows Conventional Commits
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Refactoring (no functional changes, no api changes)
• Other... Please describe:

What is the current behavior?

In some cases Signatures have more than one instance with the same name, ie multiple rects in pdf, then Merged has more than one item. Not always first item of Merged references to signature dictionary.

Example case of multiple representation of Signature1 field in PDF Document:

55 0 obj
<< /Rect [ 421 359 539 410 ] /Border [ 0 0 0 ] /T (Signature1) /F 132 /Subtype
/Widget /DA (/Helvetica 12 Tf 0 g) /AP 107 0 R /AAPL:AKExtras 108 0 R /FT
/Sig /Type /Annot /Ff 0 >>
endobj
109 0 obj
<< /Type /Annot /T (Signature1) /AP 107 0 R /Rect [ 421 359 539 410 ] /Border
[ 0 0 0 ] /Ff 0 /DA (/Helvetica 12 Tf 0 g) /F 132 /FT /Sig /Subtype /Widget
>>
endobj

Closes #

What is the new behavior?

This PR finds for Merged item which contains reference to signature dictionary (V field)

Does this PR introduce a breaking change?

• Yes
• No

Other information

[gemini-code-assist]

Summary of Changes

Hello @WitkacDevelopers, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a bug in the handling of PDF signature fields, particularly when a signature has multiple representations or instances within the document. Previously, the system might incorrectly identify the signature dictionary, leading to issues in verification or retrieval. The updated logic now intelligently searches for the correct signature dictionary among multiple merged items, ensuring accurate processing of digital signatures in complex PDF structures.

Highlights

• Improved Signature Field Handling: The pull request addresses an issue where PDF signature fields with multiple instances (e.g., multiple 'Rect' entries) were not correctly processed, especially when the first 'Merged' item did not reference the signature dictionary.
• Accurate Signature Dictionary Retrieval: The logic for retrieving the signature dictionary (keyed by '/V') from 'AcroFields' has been enhanced to correctly identify the relevant 'Merged' item by searching for the one that contains the '/V' key, rather than assuming it's always the first one.
• Robust Signature Name Detection: The 'findSignatureNames' method now also correctly identifies signature fields by searching for the appropriate 'Merged' dictionary containing the '/V' key, improving the reliability of signature detection.
• New Functional Tests: Dedicated functional tests ('PdfVerifySignatureTests.cs') have been added to ensure the correct detection and verification of signature fields, specifically for the scenario involving multiple instances, confirming the bug fix.

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[what-the-diff]

PR Summary

• Introduction of New Functional Test Class:
  A new test class called PdfVerifySignatureTests has been added. This class has methods that allow us to verify any signatures present in PDF files. It works by reading a PDF file and examining its signature fields.

• Addition of a new PDF file for testing:
  A new PDF file named issue_sig_fields.pdf has been brought in to help with testing the new functionalities.

• Improvements to existing code:
  The AcroFields.cs file, which is instrumental in handling signature fields in PDF files, has been updated. Enhancements were made to methods responsible for signature detection. In addition to this, null checks were added to prevent possible errors when a signature isn't found. The code was also refactored for increasing understanding and efficiency by specifically looking for the first dictionary containing a specific key.

[gemini-code-assist]

Code Review

This pull request successfully addresses a bug concerning the handling of signature fields with multiple instances by correctly identifying the signature dictionary. The logic is sound, and the inclusion of new functional tests is a great addition to ensure the fix is robust. I have a few suggestions to further improve the code, mainly focusing on the new test file for better readability and maintainability.