test: cover Jupyter MIME output paths

tests/test_jupyter_webapp.py

@@ -3,16 +3,30 @@ import unittest
 
 import httpx
 
+from ipython_shell.jupyter.transport import KernelMessage
 from ipython_webapp.jupyter.app import (
     InputReply,
     app,
     reply_to_input,
     run_code,
+    serialize_event,
     shell_app,
 )
 
 
 class JupyterWebAppTests(unittest.IsolatedAsyncioTestCase):
+    def test_serialize_event_encodes_binary_buffers(self):
+        event = KernelMessage(
+            msg_type="display_data",
+            parent_id="call-1",
+            content={"data": {"application/octet-stream": "present"}},
+            buffers=[b"\x00\xff"],
+        )
+
+        serialized = serialize_event(event)
+
+        self.assertEqual(serialized["buffers"], ["AP8="])
+
     async def test_run_endpoint_streams_json_events_with_mime_data(self):
         transport = httpx.ASGITransport(app=app)
         async with httpx.AsyncClient(