MessageHandlerHelper 中添加使用 Unicode 拆分字符的方法

[xbotter]

🔀 Add Unicode chunking for handling limited text

• Added a new method ChunkStringByUnicode in MessageHandlerHelper to split text into chunks based on Unicode characters.
• Added a new method TryHandleLimitedText in MessageHandlerHelper to handle long text by splitting it into chunks and processing each chunk separately.
• Added unit tests for the new methods in MessageHandlerHelperTests.

The changes enable handling of long text content by splitting it into smaller chunks based on Unicode characters. This allows for more efficient processing and handling of text content within the specified byte limit.

[xbotter]

@JeffreySu Please review it and let me know if there is anything that needs to be changed.