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/Endpoint/MetricsEndpointTest.php

@@ -0,0 +1,102 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Magdev\WpPrometheus\Tests\Unit\Endpoint;
+
+use Magdev\WpPrometheus\Endpoint\MetricsEndpoint;
+use Magdev\WpPrometheus\Metrics\Collector;
+use Magdev\WpPrometheus\Metrics\StorageFactory;
+use Magdev\WpPrometheus\Tests\Helpers\GlobalFunctionState;
+use Magdev\WpPrometheus\Tests\Unit\TestCase;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Test;
+
+#[CoversClass(MetricsEndpoint::class)]
+class MetricsEndpointTest extends TestCase
+{
+    private Collector $collector;
+    private MetricsEndpoint $endpoint;
+
+    protected function setUp(): void
+    {
+        parent::setUp();
+        $this->resetStorageFactory();
+        $this->collector = new Collector();
+        $this->endpoint = new MetricsEndpoint($this->collector);
+    }
+
+    protected function tearDown(): void
+    {
+        $this->resetStorageFactory();
+        parent::tearDown();
+    }
+
+    // ── Constructor ───────────────────────────────────────────────
+
+    #[Test]
+    public function constructor_accepts_collector(): void
+    {
+        $this->assertInstanceOf(MetricsEndpoint::class, $this->endpoint);
+    }
+
+    // ── register_endpoint() ───────────────────────────────────────
+
+    #[Test]
+    public function register_endpoint_adds_rewrite_rule(): void
+    {
+        $this->endpoint->register_endpoint();
+
+        $this->assertSame(1, GlobalFunctionState::getCallCount('add_rewrite_rule'));
+        $args = GlobalFunctionState::$callArgs['add_rewrite_rule'][0];
+        $this->assertSame('^metrics/?$', $args[0]);
+        $this->assertSame('index.php?wp_prometheus_metrics=1', $args[1]);
+        $this->assertSame('top', $args[2]);
+    }
+
+    #[Test]
+    public function register_endpoint_adds_rewrite_tag(): void
+    {
+        $this->endpoint->register_endpoint();
+
+        $this->assertSame(1, GlobalFunctionState::getCallCount('add_rewrite_tag'));
+        $args = GlobalFunctionState::$callArgs['add_rewrite_tag'][0];
+        $this->assertSame('%wp_prometheus_metrics%', $args[0]);
+        $this->assertSame('([^&]+)', $args[1]);
+    }
+
+    // ── handle_request() ──────────────────────────────────────────
+
+    #[Test]
+    public function handle_request_returns_early_when_no_query_var(): void
+    {
+        $wp = new \WP();
+        $wp->query_vars = [];
+
+        // Should return without calling exit or outputting anything.
+        $this->endpoint->handle_request($wp);
+
+        // If we reach this assertion, handle_request returned early (no exit).
+        $this->assertTrue(true);
+    }
+
+    #[Test]
+    public function handle_request_returns_early_when_query_var_empty(): void
+    {
+        $wp = new \WP();
+        $wp->query_vars = ['wp_prometheus_metrics' => ''];
+
+        $this->endpoint->handle_request($wp);
+
+        $this->assertTrue(true);
+    }
+
+    // ── Helpers ──────────────────────────────────────────────────
+
+    private function resetStorageFactory(): void
+    {
+        $reflection = new \ReflectionClass(StorageFactory::class);
+        $property = $reflection->getProperty('instance');
+        $property->setValue(null, null);
+    }
+}