deps: apply open Dependabot findings (JUnit 6, Groovy 5, GraalVM nativeimage, android-emulator-runner)

[jaime-jmebot]

Tracks #2958

Implementation plan

• Prepare the bot fork and implementation branch
• Bump org.junit:junit-bom to 6.1.3 and org.apache.groovy:groovy-test to 5.1.0 in gradle/libs.versions.toml
• Bump org.graalvm.sdk:nativeimage to 25.2.4 in jme3-nativeimage-plugin/build.gradle (svm stays at 25.0.3, upstream has no newer release)
• Bump reactivecircus/android-emulator-runner to v2.38.0 in .github/workflows/android-screenshot.yml
• Migrate the JUnit-3-style Groovy test (groovy.test.GroovyTestCase) to JUnit 5 Jupiter so it compiles and actually runs on the JUnit Platform
• Compile and run jme3-core tests (also builds the included jme3-nativeimage-plugin against nativeimage 25.2.4)
• Lint the modified workflow YAML and confirm the groovy-test 5.1.0 artifact contents
• Report final dependency versions, test results and any third-party incompatibility
• Run and validate tests
• Publish the validated patch
• Mark this pull request ready for review

This checklist is maintained automatically by Jaime.