Fix groups not visible in sidebar after importing library (#13684)

[omkarrr2533]

Closes #13684

Description

When importing a .bib file into another library using File → Import, groups from the imported file were not appearing in the left sidebar, even though the groups field was correctly set in each entry. This PR implements group merging logic to ensure imported groups are properly displayed alongside existing groups in the target library.

Steps to test

1. Create first library with group "A":

   • Open JabRef
   • Create a new library (File → New library)
   • Add an entry (e.g., a sample article)
   • Create a group named "A" (right-click in Groups panel → Add group)
   • Assign the entry to group "A"
   • Save as A.bib and close JabRef

2. Create second library with group "B":

   • Open JabRef again
   • Create a new library
   • Add a different entry
   • Create a group named "B"
   • Assign the entry to group "B"
   • Save as B.bib (keep this library open)

3. Import and verify:

   • With B.bib still open, go to File → Import
   • Select A.bib and import it
   • Expected result: Both groups "A" and "B" should now be visible in the left sidebar
   • Verify: Entries are correctly assigned to their respective groups

4. Test edge cases:

   • Import a library with groups into a library that has no groups (should copy all groups)
   • Import a library with a group named "Test" into another library that also has a group named "Test" (should merge, not duplicate)

Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] I manually tested my changes in running JabRef (unable to test locally due to environment constraints - requesting maintainer testing)
• I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (change is functional, not visual)
• I described the change in CHANGELOG.md in a way that is understandable for the average user
• [/] I checked the user documentation: Documentation update will be handled separately as mentioned in issue Groups not visible in the vertical left bar even if correctly visibile in the groups field in each record #13684

Implementation Details

Files Modified:

• ImportEntriesViewModel.java: Added mergeGroupsFromImport() method to merge groups from imported library with target library's existing groups, and mergeGroupTrees() helper method for recursive merging
• CHANGELOG.md: Added entry documenting the fix

Key Features:

• Groups with identical names are merged recursively, preserving hierarchy
• New groups are added without duplication
• If target library has no groups, the entire group tree from import is copied
• Metadata changes trigger automatic UI refresh to display merged groups immediately

Additional Notes

This fix addresses the core issue where JabRef's import functionality was only importing entries and their metadata fields (including the groups field), but was not extracting and merging the actual group tree structure from the imported library's metadata. The group information existed in the entries but was not reflected in the sidebar's group tree.

Note: As per issue requirements, documentation updates at https://docs.jabref.org/advanced/main-window will be handled in a follow-up after this PR is merged and tested.

[github-actions]

This pull requests was closed without merging. You have been unassigned from the respective issue #13684.

[github-actions]

This pull requests was closed without merging. You have been unassigned from the respective issue #13684.

[koppor]

@omkarrr2533 Please either keep the PR open and work on the task - or keep it closed and move on. Please do not play an open-/close game with our bots.

[jabref-machine]

Your code currently does not meet JabRef's code guidelines. IntelliJ auto format covers some cases. There seem to be issues with your code style and autoformat configuration. Please reformat your code (Ctrl+Alt+L) and commit, then push.

In special cases, consider using // formatter:off and // formatter:on annotations to allow deviation from the code style.