Allow server to be stopped via Ctrl + C after opening text files

[dlqqq]

Description

• Addresses Cannot stop server with Ctrl + C #41 (only fixes it after opening non-notebook files).
• Implements the stop() method on YjsClientGroup, YRoom, YRoomFileAPI, and YRoomManager.
• YRoom.stop() finishes applying all Yjs updates in the message queue, then saves the final content of the YDoc to disk.
• YRoomManager.stop() deletes all YRoom instances, calling YRoom.stop() in the process.
• Implements the stop_extension() method on RtcExtensionApp.

NOTE: the server will still hang on Ctrl + C after opening any notebook. Presumably this is due to the custom kernel & session managers we are providing.

Testing instructions

• Stop all instances of jupyter lab using kill -9 <PID>
• Run reset to reset the state of your terminal in case it was corrupted by kill -9
• Run jupyter lab and open a text file (server will still hang after opening a notebook)
• Press Ctrl + C twice and verify that the server extension stops immediately.

Demo

This also includes a build of the frontend changes from #46, which is what opens the dialog immediately when the server extension stops.

Screen.Recording.2025-05-15.at.11.46.34.AM.mov

Open questions

• @Zsailer @knaresh Are these instances supposed to stop automatically when the server extension stops, or is there some kind of method we should call in RtcExtensionApp.stop_extension()? I don't see a stop() method on these interfaces (e.g. SessionManager and AsyncKernelManager).

c.ServerApp.kernel_websocket_connection_class = "jupyter_rtc_core.kernels.websocket_connection.NextGenKernelWebsocketConnection"
c.ServerApp.kernel_manager_class = "jupyter_rtc_core.kernels.multi_kernel_manager.NextGenMappingKernelManager"
c.MultiKernelManager.kernel_manager_class = "jupyter_rtc_core.kernels.kernel_manager.NextGenKernelManager"
c.ServerApp.session_manager_class = "jupyter_rtc_core.session_manager.YDocSessionManager"

• @Zsailer @knaresh Can you all address this in a future PR? I can test with text files exclusively for now.

[dlqqq]

@jzhang20133 Thanks for the feedback! I've pushed some changes & addressed all of your feedback.

[Zsailer]

@dlqqq I can work on the cleanup for kernel client in a follow-up PR. Merging this one here!