Support polymorphism in response deserialization

[eucyt]

Releated to #728

Summary

This change introduces polymorphism support in API responses.
⚠️ This includes breaking changes.

As-Is

Polymorphic types in responses were not handled correctly.
As a result, all responses were deserialized into the base class, making it impossible to access subclass-specific properties.

$api = new MessagingApiApi($client);
$richMenuListResponse = $api->getRichMenuList();

// Even if type=postback, it becomes an instance of Action, not PostbackAction
$action = $richMenuListResponse->getRichmenus()[0]->getAreas()[0]->getAction();
// Error: Call to undefined method LINE\Clients\MessagingApi\Model\Action::getData()
$data = $action->getData();
// or simply null
$data = $action["data"];

To-Be

Polymorphic types in responses are now properly supported.
Each response is deserialized into the correct subclass, allowing subclass properties to be accessed safely.

This change applies to all API responses, not just this specific endpoint.
If your existing code depends on the base class types, you will need to update it accordingly.
You can now directly access subclass-specific properties when polymorphism is involved.

$api = new MessagingApiApi($client);
$richMenuListResponse = $api->getRichMenuList();

// If type=postback, it will now become an instance of PostbackAction
$action = $richMenuListResponse->getRichmenus()[0]->getAreas()[0]->getAction();
// You can now access subclass-specific properties
$data = $action->getData();
$data = $action["data"];

Implementation Details

The serializers are auto-generated by openapi-generator.
This fix was implemented by customizing the OpenAPI generator templates.

• 216a987 : Modified generator templates
• b174605 : Updated related tests

Not Requiring Review

• 43abd86 : Added the default template files generated by
  openapi-generator-cli author template -g php -o ./default-templates-php
  (identical to the upstream defaults)
• 48e38a6 , 859e15d : Updated auto-generated files

[eucyt]

When the model defined in OpenAPI is specified as the target class for deserialization, the process passes through here. The variable $data may contain a JSON-formatted string, an array, or an stdClass object. Therefore, we first standardize it to stdClass at this point (this has already implemented in original template).

In this PR, we recursively convert the stdClass object into an associative array and then deserialize it into $class using the fromAssocArray function, which is also used for webhook deserialization. This function supports polymorphism.

[habara-k]

Thank you for correcting my typo 😅

[Yang-33]

Q: what happens when type: unknown coms to this library? Client should not be broken even if type:unknown is came.