Migrate test suite from JUnit 4 to JUnit 5

[tgd]

Summary

• Migrated all test classes from JUnit 4 to JUnit 5
• Updated Maven dependencies (removed junit:junit, added junit-jupiter)
• Upgraded maven-surefire-plugin to 3.5.5
• Verified: test count maintained, coverage not reduced

Migration details

• Annotations: @test, @Before/@after, @BeforeClass/@afterclass, @ignore -> JUnit 5 equivalents
• Assertions: migrated to org.junit.jupiter.api.Assertions (message parameter reordered)
• Runners: @RunWith -> @ExtendWith
• Rules: converted to JUnit 5 extensions
• Exception testing: @test(expected=...) -> assertThrows()

Verification

See .migrate/ directory for baseline vs post-migration test counts and coverage data.

[sonarqubecloud]

Quality Gate passed

Issues
275 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud