test: cover Jupyter MIME output paths

src/ipython_webapp/jupyter/app.py

@@ -29,8 +29,8 @@ def serialize_event(event: KernelMessage | InputRequest) -> dict[str, object]:
             "password": event.password,
         }
 
-    # Jupyter keeps binary MIME payloads in message buffers. Base64 makes
-    # those buffers safe to carry in the same NDJSON stream as text events.
+    # Some Jupyter messages keep binary payloads in message buffers. Base64
+    # makes those buffers safe to carry beside JSON MIME data in the stream.
     return {
         "type": "kernel_message",
         "msg_type": event.msg_type,