Migrate from JUnit 4 to JUnit 5 to resolve deprecation warnings by xerial · Pull Request #897

Migrate from JUnit 4 to JUnit 5 to resolve deprecation warnings by xerial · Pull Request #897 · msgpack/msgpack-java

- Update build.sbt to use JUnit 5 dependencies (jupiter + vintage)
- Replace JUnit 4 imports with JUnit 5 equivalents
- Convert @test(expected=Exception.class) to assertThrows()
- Update @before to @beforeeach annotation
- Replace deprecated org.junit.Assert.assertThat with Hamcrest assertThat
- Maintain backward compatibility with JUnit Vintage engine

Fixes all JUnit deprecation warnings in msgpack-jackson tests.

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Co-Authored-By: Claude <noreply@anthropic.com>