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>

.gitea/workflows/release.yml

@@ -6,7 +6,32 @@ on:
       - 'v*'
 
 jobs:
+  test:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          submodules: recursive
+
+      - name: Setup PHP
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: '8.3'
+          extensions: mbstring, xml, zip, intl, gettext, redis, apcu
+          tools: composer:v2
+
+      - name: Validate composer.json
+        run: composer validate --strict
+
+      - name: Install Composer dependencies
+        run: composer install --optimize-autoloader --no-interaction
+
+      - name: Run tests
+        run: composer test
+
   build-release:
+    needs: test
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code