allow langgraph work without coversation id when checkpointing enabled

[melionel]

Description

Make conversation_id optional when checkpointer is enabled

Problem: When a LangGraph checkpointer was enabled, the adapter would throw a LangGraphMissingConversationIdError if no conversation_id was provided, blocking the request entirely.

Solution: Instead of throwing an error, the adapter generate a thread_id pass to langgraph checkpointor. When conversation_id is provided, the checkpointer continues to work as before.

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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