Code enhancements for "GUI for consistency check"

[priyanshu16095]

Follow up to #12433

Refs. https://github.com/JabRef/jabref/pull/12433/files#r1948850390
This PR updates the code to use a List instead of a String and removes a redundant toString method in ConsistencyMessage.

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 change is visible to the user)
• 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.

[HoussemNasri]

Please use a better PR title and keep the Follow up to' in PR description.

[github-actions]

Your code currently does not meet JabRef's code guidelines.
We use OpenRewrite to ensure "modern" Java coding practices.
The issues found can be automatically fixed.
Please execute the gradle task rewriteRun, check the results, commit, and push.

You can check the detailed error output by navigating to your pull request, selecting the tab "Checks", section "Tests" (on the left), subsection "OpenRewrite".

[subhramit]

Hey @priyanshu16095, is this ready?

[priyanshu16095]

Yes, its ready.