Closes #14346

Added final modifier and private constructor to StaxParser utility class to prevent instantiation and subclassing, following Java best practices for utility classes containing only static methods.

@espertusnu - This addresses the utility class refactoring discussed on Gitter.

Steps to test

1. Build the project: ./gradlew build
2. Run existing tests: ./gradlew test
3. Verify all tests pass - no functionality should be affected

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.