Refactor Langchain::Assistant::Messages::*#to_hash methods and split by roles

[andreibondarev]

Description

The current to_hash() method is lengthy and has too much logic in it. Example: https://github.com/patterns-ai-core/langchainrb/blob/main/lib/langchain/assistant/messages/openai_message.rb#L52-L80

Tasks:

• Refactor the to_hash() methods in all of Langchain::Assistant::Messages::* subclasses and split them by role. Similar to https://github.com/patterns-ai-core/langchainrb/blob/main/lib/langchain/assistant/messages/mistral_ai_message.rb#L47-L57.

[rishirajkumar97]

Hi Andrei,
I am currently working on this in this draft PR - #867
Please do lemme know if theres a update on this issue.

Regards,
Rishi J