[PHP] Enhance Symfony generator (#12532)

* Enhance Symfony generator

  - Add missing typehints
  - Add missing use statements
  - Simplify model ctor
  - Change fallthrough Exception to Throwable
  - prevent storing result of void methods
  - fix validate method
  - add default null values to model properties
  - simplify API interface return values
  - fix/rework default value implementation
  - fix optional params deprecation warnings
  - ..

For more details check out the PR: https://github.com/OpenAPITools/openapi-generator/pull/12532

* Enhance Symfony generator Tests

  - Skip risky tests
  - Fix type hint error
  - Fix class exists tests
  - Fix broken doc block
  - Enhance annotations
  - Update phpunit.xml.dist
  - Fix test config resource location

samples/server/petstore/php-symfony/SymfonyBundle-php/Tests/AppKernel.php

@@ -8,15 +8,20 @@ use Symfony\Component\Config\Loader\LoaderInterface;
 
 class AppKernel extends Kernel
 {
+    /**
+     * @inheritDoc
+     */
     public function registerBundles(): iterable
     {
-        $bundles = array(
-            new FrameworkBundle()
-        );
-
-        return $bundles;
+        return [
+            new FrameworkBundle(),
+        ];
     }
 
+    /**
+     * @inheritDoc
+     * @throws \Exception
+     */
     public function registerContainerConfiguration(LoaderInterface $loader)
     {
         $loader->load(__DIR__.'/test_config.yml');