Ensure first chat message is retained to prevent "Could not determine…

… chat role" error (#633)

sgpt/handlers/chat_handler.py

@@ -71,7 +71,9 @@ def _read(self, chat_id: str) -> List[Dict[str, str]]:
 
     def _write(self, messages: List[Dict[str, str]], chat_id: str) -> None:
         file_path = self.storage_path / chat_id
-        json.dump(messages[-self.length :], file_path.open("w"))
+        # Retain the first message since it defines the role
+        truncated_messages = messages[:1] + messages[1 + max(0, len(messages) - self.length):]
+        json.dump(truncated_messages, file_path.open("w"))
 
     def invalidate(self, chat_id: str) -> None:
         file_path = self.storage_path / chat_id