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/Helpers/GlobalFunctionState.php

@@ -0,0 +1,51 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Magdev\WpPrometheus\Tests\Helpers;
+
+/**
+ * Controllable state for global WordPress function stubs.
+ *
+ * Used by the bootstrap's global get_option() stub so that tests
+ * for global-scope functions (e.g., wp_prometheus_authenticate_request)
+ * can control return values without php-mock (which requires namespaces).
+ */
+class GlobalFunctionState
+{
+    /** @var array<string, mixed> Simulated WordPress options. */
+    public static array $options = [];
+
+    /** @var array<string, int> Track function call counts. */
+    public static array $callCounts = [];
+
+    /** @var array<string, list<mixed>> Track arguments passed to functions. */
+    public static array $callArgs = [];
+
+    /**
+     * Reset all state. Call in setUp()/tearDown().
+     */
+    public static function reset(): void
+    {
+        self::$options = [];
+        self::$callCounts = [];
+        self::$callArgs = [];
+    }
+
+    /**
+     * Record a function call for later assertions.
+     */
+    public static function recordCall(string $function, mixed ...$args): void
+    {
+        self::$callCounts[$function] = (self::$callCounts[$function] ?? 0) + 1;
+        self::$callArgs[$function][] = $args;
+    }
+
+    /**
+     * Get call count for a function.
+     */
+    public static function getCallCount(string $function): int
+    {
+        return self::$callCounts[$function] ?? 0;
+    }
+}