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>

CHANGELOG.md

@@ -5,6 +5,27 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.5.0] - 2026-02-26
+
+### Added
+
+- Comprehensive PHPUnit test suite with 189 tests and 329 assertions:
+  - CustomMetricBuilderTest (35 tests) - validation, CRUD, import/export
+  - AuthenticationTest (13 tests) - Bearer token, query param, header extraction
+  - StorageFactoryTest (25 tests) - adapter config, env vars, connection testing
+  - RuntimeCollectorTest (22 tests) - endpoint normalization, histograms, singleton
+  - DashboardProviderTest (27 tests) - registration validation, path traversal security
+  - InstallerTest (11 tests) - activation, deactivation, uninstall cleanup
+  - CollectorTest (10 tests) - registry, metric registration, render output
+  - MetricsEndpointTest (5 tests) - rewrite rules, request routing
+- Test bootstrap with WordPress function stubs and GlobalFunctionState helper
+- CI/CD test job in Gitea release workflow that gates release builds
+- php-mock/php-mock-phpunit dependency for mocking WordPress functions in namespaced code
+
+### Changed
+
+- Release pipeline now requires passing tests before building release packages
+
 ## [0.4.9] - 2026-02-26
 
 ### Security