Add PHPUnit test suite and CI workflow

[martinydeAI]

Goal

Provide a runnable PHPUnit test suite and a CI workflow so PRs run tests
on every change.

Background

Issue #10 (Set up GitHub Actions for coding standards and tests) bundled
two halves. The coding-standards half landed in #4 (eight workflow files
from the symfony-8 template). The test-suite half was never started —
no phpunit dependency, no phpunit.xml, no tests/ directory, no
PHPUnit workflow. This issue captures only that remaining test-suite
work.

The upstream
itk-dev/devops_itkdev-docker
template repo does not ship a PHPUnit workflow template for either
Symfony or Drupal (only php.yaml / twig.yaml / styles.yaml /
javascript.yaml). So the workflow we add here is a project-local
file and will not carry the upstream "Do not edit this file!" header.
An optional follow-up is to contribute it upstream — out of scope here.

The App\Tests\ PSR-4 namespace is already reserved in composer.json
(autoload-dev points at tests/), so the wiring is half there.

Steps

1. Install the test runner inside the container:

   itkdev-docker-compose composer require --dev symfony/test-pack

   This brings in phpunit/phpunit, symfony/phpunit-bridge, and the
   Symfony recipe installs phpunit.xml.dist and tests/bootstrap.php.

2. Commit the recipe-installed files (phpunit.xml.dist,
   tests/bootstrap.php, updated symfony.lock, .env.test if added).

3. Add a trivial smoke test at tests/SmokeTest.php so CI has something
   real to run.

4. Verify locally:

   itkdev-docker-compose vendor/bin/phpunit

5. Add .github/workflows/phpunit.yaml modelled on the existing
   php.yaml. Trigger on **/*.php, composer.json, composer.lock,
   docker-compose.yml, phpunit.xml*. Inside the job:

   docker compose run --rm phpfpm composer install
   docker compose run --rm phpfpm vendor/bin/phpunit

6. Update CHANGELOG.md under [Unreleased] (Added).

Acceptance criteria

• phpunit/phpunit (via symfony/test-pack or directly) is in
  require-dev.
• phpunit.xml.dist and at least one passing test under tests/
  are committed.
• itkdev-docker-compose vendor/bin/phpunit exits 0 locally.
• .github/workflows/phpunit.yaml runs the test suite on PRs and
  pushes to main / develop.
• CI reports green on a fresh PR.
• CHANGELOG.md updated.
• Reviewed and merged via PR.

Optional follow-up (not blocking)

• Open a PR against itk-dev/devops_itkdev-docker adding
  github/workflows/symfony/phpunit.yaml so future Symfony projects
  pick it up automatically.

References

• Closed-out portion of Set up GitHub Actions for coding standards and tests #10.
• Project conventions: CLAUDE.md (PR docs: add CLAUDE.md and refocus README as human-facing docs #30).
• Existing workflow style: .github/workflows/php.yaml.