Duplicate check on importing in find unlinked files

[R-Zhou4]

This PR resolves the issue of duplicate entries being imported when clicking the "Import" button multiple times by refining the import process.
"Closes koppor#551".

Key Changes:

UnlinkedFilesDialogViewModel Class:

• edit startImport Method:

  • Import Logic: Uses the ImportHandler.getEntriesToImport method to retrieve a list of entries.
  • Deduplication Logic: Prevents duplicate entries from being imported by using the importHandler.findDuplicate method.

• add getFilePathFromEntry Method:

  • Extracts the correct file path from BibEntry.
  • Returns a valid path, or an empty path if no valid fields are found.

ImportHandler Class:

• add getEntriesToImport Method:
  • Processes the incoming list of files and extracts importable BibEntry objects.
  • Handles different file types (e.g., PDF and BibTeX) and returns unique entries.
  • Maintains import integrity by creating empty entries for unsupported file types.

BibEntry Class:

• add getCiteKey Method:
  • Adds a method for retrieving the citation key to support duplicate entry detection.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[koppor]

Small initial comment while on the road.

[koppor]

Remove. Use

jabref/src/main/java/org/jabref/model/entry/BibEntry.java

Line 403 in 4ce83c4

public Optional<String> getCitationKey() {

Move method below up, too.

[R-Zhou4]

Updated: getCiteKey() has been removed.

[Siedlerchr]

You can use a lambda here directly no need for anonymous method, Intellij should offer you that already

[R-Zhou4]

Replaced with a lambda now