[php-nextgen]: Fix imports for models with parent schema (#23317)

Co-authored-by: coffeemakr <coffeemakr@users.noreply.github.com>

Author: coffeemakr

modules/openapi-generator/src/main/resources/php-nextgen/model.mustache

@@ -21,14 +21,14 @@
 
 namespace {{modelPackage}};
 {{^isEnum}}
-{{^parentSchema}}
 
+{{^parentSchema}}
 use ArrayAccess;
 use JsonSerializable;
+{{/parentSchema}}
 use InvalidArgumentException;
 use ReturnTypeWillChange;
 use {{invokerPackage}}\ObjectSerializer;
-{{/parentSchema}}
 {{/isEnum}}
 
 /**

samples/client/echo_api/php-nextgen-streaming/src/Model/DataQuery.php

@@ -28,6 +28,10 @@
 
 namespace OpenAPI\Client\Model;
 
+use InvalidArgumentException;
+use ReturnTypeWillChange;
+use OpenAPI\Client\ObjectSerializer;
+
 /**
  * DataQuery Class Doc Comment
  *

samples/client/echo_api/php-nextgen/src/Model/DataQuery.php

@@ -28,6 +28,10 @@
 
 namespace OpenAPI\Client\Model;
 
+use InvalidArgumentException;
+use ReturnTypeWillChange;
+use OpenAPI\Client\ObjectSerializer;
+
 /**
  * DataQuery Class Doc Comment
  *

samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Model/Cat.php

@@ -27,6 +27,10 @@
 
 namespace OpenAPI\Client\Model;
 
+use InvalidArgumentException;
+use ReturnTypeWillChange;
+use OpenAPI\Client\ObjectSerializer;
+
 /**
  * Cat Class Doc Comment
  *

samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Model/ChildWithNullable.php

@@ -27,6 +27,10 @@
 
 namespace OpenAPI\Client\Model;
 
+use InvalidArgumentException;
+use ReturnTypeWillChange;
+use OpenAPI\Client\ObjectSerializer;
+
 /**
  * ChildWithNullable Class Doc Comment
  *

samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Model/DiscriminatorChild.php

@@ -27,6 +27,10 @@
 
 namespace OpenAPI\Client\Model;
 
+use InvalidArgumentException;
+use ReturnTypeWillChange;
+use OpenAPI\Client\ObjectSerializer;
+
 /**
  * DiscriminatorChild Class Doc Comment
  *

samples/client/petstore/php-nextgen/OpenAPIClient-php/src/Model/Dog.php

@@ -27,6 +27,10 @@
 
 namespace OpenAPI\Client\Model;
 
+use InvalidArgumentException;
+use ReturnTypeWillChange;
+use OpenAPI\Client\ObjectSerializer;
+
 /**
  * Dog Class Doc Comment
  *