test(log4j2): Add initial test suite covering plugin registration

[rschmitt]

This package previously had no tests and no JUnit dependency. Add JUnit Jupiter, the surefire plugin, and an initial test class that exercises the full plugin resolution path end-to-end: a log4j2.xml on the test classpath, a real LogManager-issued logger, and stdout capture (LambdaAppender writes through LambdaRuntime.getLogger() to System.out).

The TEXT test routes the root logger through LambdaTextFormat with a deterministic PatternLayout and asserts each level appears. The JSON test adds a second LambdaAppender with format="JSON" backed by JsonTemplateLayout + LambdaLayout.json, attached via additivity=false to a "json-test" logger, and asserts the messages show up JSON-encoded.

Add log4j-layout-template-json at test scope so the JSON path can resolve at test time. Users are still expected to bring their own copy at runtime (peer-dependency model, like PatternLayout); the published artifact's dependency surface is unchanged.

The tests succeed on Java 8 and fail on Java 25, because annotation processors are not run by default on Java 25, Log4j2Plugins.dat is not generated for our plugins, and both tests surface the resulting CLASS_NOT_FOUND from the log4j status logger.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[darklight3it]

This should be merged before #616

that is needed to prevent situation like #612

[darklight3it]

LGTM 🚀 Thanks for the help!