Add PHPUnit test suite

- Add PHPUnit 11.0 as dev dependency
- Add phpunit.xml configuration
- Add DTO tests (LicenseInfo, LicenseStatus, ActivationResult)
- Add Exception tests (factory method, all exception types)
- Add LicenseClient tests with mocked HTTP responses
- Update README with testing instructions
- Update CHANGELOG

32 tests, 93 assertions

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

composer.json

@@ -22,6 +22,9 @@
         "psr/log": "^3.0",
         "symfony/http-client": "^7.0"
     },
+    "require-dev": {
+        "phpunit/phpunit": "^11.0"
+    },
     "autoload": {
         "psr-4": {
             "Magdev\\WcLicensedProductClient\\": "src/"