ci: add JUnit sample test workflow

[outofcoffee]

Adds a GitHub Actions workflow that runs the junit-sample integration test against imposter-jvm-engine's main, mirroring the check that lives in the engine repo today.

Summary

• Add .github/workflows/junit-sample.yaml: checks out this repo and imposter-project/imposter-jvm-engine, publishes the engine to a local Maven repo (./gradlew publishToMavenLocal -xtest), then runs ./mvnw test against the locally-published SNAPSHOT
• Triggers: push to main, pull requests, and manual workflow_dispatch
• Publishes JUnit results via EnricoMi/publish-unit-test-result-action so failures surface as checks

Implementation details

This is a near-mirror of the existing Examples job in imposter-jvm-engine. Both jobs end up testing the same combination of sample + engine, but having it here lets contributions to the examples repo block on the same check before merge — previously, a breaking change to a sample pom (like the Jackson version drift fixed in #5) would only be caught by the engine's CI on its next run.

The job pins Java 11 to match the engine's build toolchain; setup-java's cache: 'maven' covers the sample's mvnw downloads. The 15-minute timeout gives Gradle's first-time publishToMavenLocal enough headroom on a cold cache.

Stacked on top of #5 — re-targets main once that merges.