devcontainer: Update Java, Maven, and JUnit

[Christof Marti (chrmarti)]

Summary

Refresh this getting-started sample to the newest stable runtime available in the official Dev Container images, and keep its project settings and documentation consistent.

• Move from Java 21 / Debian 12 to the official Java 25 / Debian 13 image (java:3-25-trixie).
• Update Maven to 3.10.0, compiler plugin to 3.16.0, Surefire to 3.6.0, and tests to JUnit Jupiter 6.1.3.
• Align compiler release, Eclipse settings, and CI with Java 25.
• Include the CLI-generated Java Feature lockfile and document the toolchain.

Validation

Dev Container CLI 0.89.0 startup/exec; Java 25.0.4.1 and Maven 3.10.0; mvn verify passed with 2 tests, no failures/errors/skips; expected application output; JAR contents and JDWP debug-agent startup.

The relevant build/syntax/lint checks were rerun in the dev container before creating this PR. Images were freshly pulled and rechecked on 2026-10-10. Validation was on linux/amd64 using Docker Desktop on Windows; this is not a claim of full VS Code UI or multi-architecture testing.

Reviewer notes

The freshly pulled official Java image does not include slirp4netns or iptables. These packages are not added by this PR. Local validation used Microsoft OpenJDK; CI remains on Temurin with its version updated to 25.

[Connor Peet (connor4312)]

Reviewed the complete diff and relevant build, container, editor, test, and debugger context for security and correctness. No actionable P1/P2 findings. Java 25 targeting and the JUnit migration are consistent; the reported build checks pass. No local repository changes were made.

_{Written by Connor's agent running GPT-6.1 Sol}