Add PHPUnit unit tests for plugin classes (0% → 98% line coverage)

[devin-ai-integration]

Summary

The repo had no tests at all — coverage of src/ was 0%. This adds a PHPUnit suite that runs without a WordPress installation, covering all four classes (Plugin, Frontend, Admin, REST): 47 tests, 100% of classes/methods and 97.8% of lines (the only uncovered lines are the exit in each ABSPATH guard).

Instead of pulling in WP test scaffolding, tests/wp-stubs.php defines the ~40 WP functions the plugin calls plus WP_Query/WP_Error doubles, and WPStub records every call so tests can assert on registration side effects:

new REST( $pluginStub );
$call = WPStub::firstCall( 'register_rest_route' ); // ['doneproject/v1', '/hello', [...]]

Most of the coverage weight is Frontend, where the interesting logic lives — the two private helpers are exercised via reflection: get_first_image_src_from_content (no img, single/double quoted src, first-of-many) and generate_content_image_thumbnail across every branch: remote URL passthrough, missing local file, query-string stripping, cached thumb reuse (no editor call), wp_mkdir_p failure, wp_get_image_editor returning WP_Error, and save() failure. Thumbnail tests write real files into a temp uploads dir so file_exists/md5_file behave as in production.

Two supporting fixes:

• composer.json PSR-4 prefix was over-escaped ("DoneProject\\\\" → literal DoneProject\\), so Composer autoloading never resolved the classes; corrected to "DoneProject\\" and added autoload-dev for DoneProject\Tests\ plus composer test scripts.
• .gitignore now excludes /vendor/, /composer.lock, .phpunit.result.cache.

Also adds a Tests GitHub Actions workflow running the suite on PHP 7.4/8.1/8.3 (composer update per version, hence no committed lock) and a README section on running tests/coverage.

Link to Devin session: https://app.devin.ai/sessions/15517074725a4224986972cd3c154942
Requested by: @liikane

[devin-ai-integration]

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