Refactor CitationKeyGeneratorTest to use parameterized tests

[Zeglow]

Refs JabRef#676

Refactored CitationKeyGeneratorTest to use parameterized tests with @ParameterizedTest and @MethodSource, reducing code duplication while maintaining test coverage.

Collaboration: This refactoring was completed in collaboration with my classmate @hisunll. We divided the work - she refactored the first half of the test class, and I refactored the second half.

Note: My classmates are also contributing to fixing this issue as part of our CS5010 coursework. cc @espertusnu

Steps to test

This is a test refactoring PR with no functional changes to the application.

1. Run the test suite:

   ./gradlew test --tests CitationKeyGeneratorTest

2. Verify all tests pass
3. Review the refactored code to ensure:
   • All original test cases are preserved
   • Parameterized tests are properly structured
   • Method sources correctly provide test data
4. Optionally, compare the test coverage before and after to confirm no regression

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 change is visible to the user)
• [/] I described the change in CHANGELOG.md in a way that is understandable for the average user (if change is visible to the user)
• [/] I checked the user 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 updating file(s) in https://github.com/JabRef/user-documentation/tree/main/en.