ci: verify staged artifacts with external Maven sample

[omer-cengel]

Summary

Add an isolated verification path so pull-request CI proves that the packaged CLI and runtime work from a clean Java 21 Maven/PostgreSQL consumer.

Changes

• Add an independent Maven sample covering the documented five-query CRUD workflow and caller-owned commit and rollback.
• Stage reactor artifacts in an isolated Maven repository and verify CLI version, handled failures, generated sources, dependency isolation, compilation, and execution.
• Make unavailable Docker fail PostgreSQL integration coverage when CI explicitly requires it while preserving local skip behavior.
• Run the full test suite and external sample against a health-checked PostgreSQL 16 service.

Scope and non-goals

This does not alter compiler behavior, runtime APIs, generated APIs, or supported SQL. It does not add lifecycle-bound generation plugins, public publication, signing, checksums, release metadata, or user-documentation changes.