Migrate Java tests from JUnit 4 to JUnit 5 Jupiter

[benfortuna]

Summary

The Java test suite was written in JUnit 4 style but the build runs with useJUnitPlatform() and only Jupiter on the test runtime classpath. With no vintage engine registered, JUnit Platform was silently skipping every JUnit 4 test class at discovery time — most of the vCard property and parameter coverage was effectively offline.

This PR completes the full migration to JUnit 5 Jupiter. The vintage engine is added as a temporary bridge, all test sources are migrated, and vintage is then removed.

Key changes

• 67 test classes rewritten to use only org.junit.jupiter.api.* and org.junit.jupiter.params.*
• PropertyTest and ParameterTest become abstract @ParameterizedTest hosts; subclasses now supply only public static Stream<Arguments> parameters()
• New ExtensionPropertyTest and ExtensionParameterTest preserve the base classes' own parameter rows
• 6 subclasses with their own @Test methods needing extra args (BDay, Member, Revision, Source, Uid, Url) use a dual-@MethodSource pattern
• Misleading junitVintage version key renamed to junit
• Pre-existing @Ignore usages converted to @Disabled

Verification

• ./gradlew clean test → BUILD SUCCESSFUL: 1101 tests, 0 failures, 0 errors, 13 skipped
• Active test parity: 1091 active tests pre- and post-migration
• Grep gate: zero org.junit.{Test,Assert,Before,After,Ignore,runner,runners} imports remain
• Vintage engine removed from testRuntimeClasspath

Notes

• Test source classes are not published in the main jar, so external impact from the abstract-base reshape is unlikely.
• Groovy / Spock specs were unaffected (they already ran via spock-core on JUnit Platform).
• See openspec/changes/migrate-junit4-tests-to-junit5/ for the full proposal, design, and per-phase task breakdown.

Test plan

• CI green on this branch
• Verify reports under build/test-results/test/ include every JUnit 4 class that was previously missing (spot check AgentTest, PropertyTest subclasses)
• Confirm no JUnit 4 imports remain: grep -rE "org\.junit\.(Test|Assert|Before|After|Ignore|runner|runners)\b" src/test/java

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