build: update Java dependencies - #18

Summary

• update protobuf, Jackson, JUnit, SnakeYAML Engine, and Spotless to current stable releases
• update example applications within their existing supported framework generations
• retain AssertJ and Maven plugin versions when the only available updates are milestones or Maven 4 betas

Validation

• Maven verify on Java 21 with 105 unit tests and 54 fake-engine integration tests
• coverage threshold and Spotless checks
• all ten example projects compile against the updated SDK snapshot
• actionlint
• gitleaks and repository identity guard

The eight real-engine integration tests remain opt-in and were not enabled locally.