Migrate from JUnit 4 to JUnit 5

[basmussen]

Summary

• Migrate testing framework from JUnit 4 to JUnit 5
• Update test assertions to use modern JUnit 5 API

Changes

• Replace junit 4.13.2 dependency with junit-jupiter 5.11.4
• Update all @test(expected=Exception.class) annotations to use assertThrows
• Add necessary imports for JUnit 5 assertions
• Improve test structure and readability

Testing

All 86 tests pass successfully with JUnit 5.

Closes #29