feat: Add comprehensive PHPUnit test suite and CI/CD test gating (v0.5.0)

189 tests across 8 test classes covering all core plugin classes:
CustomMetricBuilder, StorageFactory, Authentication, DashboardProvider,
RuntimeCollector, Installer, Collector, and MetricsEndpoint.

Added test job to Gitea release workflow that gates build-release.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

composer.json

@@ -32,6 +32,7 @@
         "promphp/prometheus_client_php": "^2.10"
     },
     "require-dev": {
+        "php-mock/php-mock-phpunit": "^2.10",
         "phpunit/phpunit": "^10.0",
         "squizlabs/php_codesniffer": "^3.7",
         "wp-coding-standards/wpcs": "^3.0",