test(fields): add PHPUnit test suite for plugin

• New phpunit.xml and bootstrap for the Fields plugin
• PluginFieldsContainerTest covers:
  - Creating / reading containers
  - Filtering containers by entity, subtype, recursion
  - Hooks: preItem, postItemAdd, preItemUpdate
  - populateData (single + multi-dropdown)
  - showForTab rendering and rights checks

Author: JeremieMercier

.gitignore

@@ -2,3 +2,4 @@ dist/
 vendor/
 .gh_token
 *.min.*
+*.cache

composer.json

@@ -9,7 +9,8 @@
         "php-parallel-lint/php-parallel-lint": "^1.4",
         "phpstan/extension-installer": "^1.4",
         "phpstan/phpstan": "^2.1",
-        "phpstan/phpstan-deprecation-rules": "^2.0"
+        "phpstan/phpstan-deprecation-rules": "^2.0",
+        "phpunit/phpunit": "^9.6"
     },
     "config": {
         "optimize-autoloader": true,