[FR] Open up FirebaseMessagingException

**Is your feature request related to a problem? Please describe.**
FirebaseMessagingException is final and has package private/private constructors. This prevents using the exception in tests. The firebase recommendation to deal with invalid tokens is to react on error code UNREGISTERED and INVALID_ARGUMENT. In order to test my code that deals with this scenario I would need to create a FirebaseMessagingException with the specified error codes which I can't.

**Describe the solution you'd like**
Open up FirebaseMessagingException to allow instantiation in tests.

首先定位 FirebaseMessagingException，並檢查其建構函式和現有測試。確認測試可以為錯誤碼 UNREGISTERED 和 INVALID_ARGUMENT 建立例外實例，同時保留 SDK 預期的例外行為；當這些情境可測試時即完成。

由索引模型根據 Issue 內容生成。