`metadata` argument is not passed to convert laggraph message

[amarmudrankit]

[ ] I have checked the documentation and related resources and couldn't resolve my bug.

Describe the bug
When converting langgraph messages into ragas messages, if the user requests to include metadata, it is not copied for AIMessage. Works for Human and Tool message because, MESSAGE_TYPE_MAP gets set at the beginning of the method.

Particularly this line needs to be modified to send metadata argument received

Ragas version: 0.2.15
Python version: 3.12

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