Uses Pattern.UNICODE_CHARACTER_CLASS so that \w also matches non-ASCII characters (e.g. Cyrillic, Chinese) in citation keys.
Adds unit tests with Cyrillic keys.

Closes #<13301>

Summary of changes

• ReferenceMark.java
  Compiles both regex patterns with Pattern.UNICODE_CHARACTER_CLASS, allowing Unicode citation keys.
• ReferenceMarkTest.java
  Adds two test cases (Ты2025, Я2025) to verify parsing of Cyrillic keys.
• Manual verification in LibreOffice Writer confirms bibliography generation now works with non-Latin keys.

Steps to test

1. Build & launch JabRef from this branch

2. In JabRef, create a new library containing:

   @Article{Ты2025,
     author = {Ты},
     title  = {You},
     year   = {2025},
   }
   
   @Article{Я2025},
     author = {Я},
     title  = {I},
     year   = {2025},
   }

3. Connect to an open LibreOffice Writer document via the OO/LO panel.

4. Insert citations for both entries, then click Make/Sync bibliography.
   Expected: bibliography appears without error; Writer’s field names show in LibreOffice Writer.

5. Run automated tests

   ./gradlew jablib:test --tests org.jabref.logic.openoffice.ReferenceMarkTest

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user “We fixed an issue where the LibreOffice integration did not support citation keys containing Unicode characters.”
• Tests created for changes
• Manually tested changed features in running JabRef
• Screen Recording added in PR description.
• Checked developer's documentation: N/A (no docs change needed)
• Checked user documentation: opened issue / PR if docs need updating