Parent class modifications are lost when both parent and child are modified via Extension

## Description

* Using Extension to modify both a parent class and its subclass will cause the modifications to the parent class to become completely ineffective.
* The call order of `install_script_extension` does not affect the result — the modifications to the parent class always become ineffective.
* The modifications to the subclass remain effective.
* No error log.
* If one side or both are modified through Hook instead, this issue will not occur.

## Environment

* OS: Windows 11
* Game: Dome Keeper
* Game Engine: godotsteam v4.2.2