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>

README.md

@@ -222,6 +222,46 @@ add_action( 'wp_prometheus_register_dashboards', function( $provider ) {
 
 ## Development
 
+### Testing
+
+The plugin includes a comprehensive PHPUnit test suite with 189 tests covering all core classes.
+
+```bash
+# Install dependencies (including dev)
+composer install
+
+# Run the full test suite
+composer test
+```
+
+#### Test Architecture
+
+Tests use [php-mock/php-mock-phpunit](https://github.com/php-mock/php-mock-phpunit) to mock WordPress functions called from namespaced plugin code, and a `GlobalFunctionState` helper for controlling global-scope function stubs (authentication, options, conditionals).
+
+```
+tests/
+├── bootstrap.php                         # WP constants + function stubs
+├── Helpers/
+│   └── GlobalFunctionState.php           # Controllable state for stubs
+└── Unit/
+    ├── TestCase.php                      # Base class with PHPMock trait
+    ├── AuthenticationTest.php            # Bearer/query auth, header extraction
+    ├── InstallerTest.php                 # Activate, deactivate, uninstall
+    ├── Metrics/
+    │   ├── CustomMetricBuilderTest.php   # Validation, CRUD, import/export
+    │   ├── RuntimeCollectorTest.php      # Endpoint normalization, histograms
+    │   ├── StorageFactoryTest.php        # Adapter config, env vars, connections
+    │   └── CollectorTest.php             # Registry, register_*, render
+    ├── Admin/
+    │   └── DashboardProviderTest.php     # Registration, path security
+    └── Endpoint/
+        └── MetricsEndpointTest.php       # Rewrite rules, request routing
+```
+
+#### CI/CD Integration
+
+Tests run automatically in the Gitea CI/CD pipeline before release builds. A failing test suite blocks the release.
+
 ### Build for Release
 
 ```bash