test: add test workflow support and initial tests

[rbcorrales]

Summary

1. Adds PHPUnit coverage for the shared version bump helper.
2. Adds a reusable test-command input to CI and release workflows.
3. Tightens release zip verification for test and development artifacts.

Testing

1. composer validate --strict
2. composer lint
3. composer test

[Copilot]

Pull request overview

Adds PHPUnit-based testing infrastructure to the repository (initial unit tests for the version bump helper, bootstrap, config), threads a new test-command input through the reusable CI and release workflows, and rewrites the release-zip forbidden-content verification to use explicit loops with broader forbidden file/dir coverage.

Changes:

• Refactor scripts/bump-plugin-version.php to expose bump_plugin_version() as library-callable (entrypoint guard + CLI wrapper) and add PHPUnit + tests/bootstrap covering bump, prerelease rejection, and ambiguous-header failure.
• Add test-command input to plugin-ci.yml and plugin-release.yml, run composer test in repo's own ci.yml, and document the new input in README/AGENTS.
• Tighten release zip verification: expand forbidden entries (tests, scripts, stubs, dev configs, docs, caches) and switch from ! grep -qE one-liners to explicit while/exit 1 loops; pin Composer platform PHP to 8.3.0 and update lockfile with PHPUnit 12 dep tree.

Reviewed changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
scripts/bump-plugin-version.php	Split CLI bootstrapping from bump_plugin_version() so it can be required from tests without executing.
tests/Unit/BumpPluginVersionTest.php	New PHPUnit coverage for happy path, prerelease rejection, and ambiguous header.
tests/bootstrap.php	Requires the bump script for the test suite.
phpunit.xml.dist	PHPUnit 12 config pointing at tests/Unit.
composer.json / composer.lock	Add phpunit/phpunit ^12.0, composer test script, pin platform PHP 8.3.0.
.github/workflows/plugin-ci.yml	Add optional test-command input executed before build.
.github/workflows/plugin-release.yml	Add test-command input; rewrite forbidden-entry/src checks with explicit loops and a broader forbidden list.
.github/workflows/ci.yml	Run composer test after lint in repo CI.
README.md / AGENTS.md	Document test-command input and PHPUnit as a local check surface.
.gitignore	Ignore .phpunit.cache/.

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