feat: define Jupyter transport message boundary

tests/test_transport.py

@@ -0,0 +1,21 @@
+import unittest
+
+from ipython_shell.transport import KernelMessage
+
+
+class TransportTests(unittest.TestCase):
+    def test_kernel_message_keeps_content_and_buffers(self):
+        message = KernelMessage(
+            msg_type="display_data",
+            parent_id="call-1",
+            content={"data": {"text/plain": "42"}},
+            buffers=[b"binary"],
+        )
+
+        self.assertEqual(message.parent_id, "call-1")
+        self.assertEqual(message.content["data"], {"text/plain": "42"})
+        self.assertEqual(message.buffers, [b"binary"])
+
+
+if __name__ == "__main__":
+    unittest.main()