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>

phpunit.xml

@@ -0,0 +1,22 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="vendor/phpunit/phpunit/phpunit.xsd"
+         bootstrap="vendor/autoload.php"
+         colors="true"
+         cacheDirectory=".phpunit.cache"
+         executionOrder="depends,defects"
+         beStrictAboutOutputDuringTests="true"
+         failOnRisky="true"
+         failOnWarning="true">
+    <testsuites>
+        <testsuite name="default">
+            <directory>tests</directory>
+        </testsuite>
+    </testsuites>
+
+    <source>
+        <include>
+            <directory>src</directory>
+        </include>
+    </source>
+</phpunit>