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>

tests/Unit/TestCase.php

@@ -0,0 +1,32 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Magdev\WpPrometheus\Tests\Unit;
+
+use Magdev\WpPrometheus\Tests\Helpers\GlobalFunctionState;
+use phpmock\phpunit\PHPMock;
+use PHPUnit\Framework\TestCase as PHPUnitTestCase;
+
+/**
+ * Base test case for WP Prometheus unit tests.
+ *
+ * Provides the PHPMock trait for mocking WordPress functions
+ * called from namespaced code, and resets global state between tests.
+ */
+abstract class TestCase extends PHPUnitTestCase
+{
+    use PHPMock;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        GlobalFunctionState::reset();
+    }
+
+    protected function tearDown(): void
+    {
+        GlobalFunctionState::reset();
+        parent::tearDown();
+    }
+}