Fix message duplication in GraphState messages

[hamidriasat]

Context:

In the GraphState class, the messages variable is of type list, annotated with add_messages. This ensures that new messages are appended to the list without needing to resend old messages, as they are already present.

Problem:

In the generate and code_check methods, the existing list was being extracted and, after appending a new message, sent back to update the messages variable. This approach caused duplication since the old messages were already present in the list. The correct behavior should only send back the new message to be appended.

Solution:

This pull request fixes the issue by ensuring that only the new message is sent from the generate and code_check methods, preventing duplication.

Additional Notes:

• I verified the issue through debugging the GraphState variables.
• The rest of the script remains unchanged.