Remove SmartGroup and refactor groups factory

[shubhamk0205]

Closes #14143

This PR removes the SmartGroup class and replaces it with ExplicitGroup, as both provided the same functionality. It also resolves an architectural violation by moving the group factory methods from the GUI layer (JabRefSuggestedGroups) into the logic layer (GroupsFactory). Additionally, DefaultGroupsFactory has been renamed to GroupsFactory for improved clarity and naming consistency.

To preserve compatibility, migration logic has been added to ensure that existing .bib files containing serialized SmartGroup entries continue to load correctly and are transparently converted to ExplicitGroup.

Steps to test

1. Test group creation and functionality

   • Open JabRef and create a new library.
   • Right-click on the All entries group → Add JabRef suggested groups.
   • Verify that:
     • Entries without linked files
     • Entries without groups
       are created successfully.
   • Ensure both groups filter entries as expected.

2. Test backward compatibility

   • Open a .bib file containing SmartGroup entries (if available).
   • Verify that the groups load correctly and appear in the group tree.
   • Confirm that SmartGroup entries are automatically migrated to ExplicitGroup.

3. Test “Imported Entries” group

   • Enable Preferences → Library → Add imported entries to group.
   • Import entries using web search, PDF import, or fetchers.
   • Check that imported entries appear in the Imported Entries group.
   • Confirm the group is an ExplicitGroup.

4. Test general group operations

   • Create, edit, and delete groups.
   • Drag and drop groups in the groups tree.
   • Ensure all group operations behave normally without regressions.

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 (always required)
• [/] I added JUnit tests for changes (if applicable)
• [/] I added screenshots in the PR description (if the change is visible to the user)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if the change is visible to the user)
• [/] I checked the user documentation: If documentation needs updates, I opened an issue or submitted a PR to https://github.com/JabRef/user-documentation/

[github-actions]

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[Siedlerchr]

Please also add a test for the migration of the SmartGroup to the new Explicit groups and ideally roundtrip test.
Reading in old format -> Conversion -> Storing in new format

[Siedlerchr]

add tests for parsing and roundtrip

[shubhamk0205]

Done with the tests @Siedlerchr