Handle YChat document resets when jupyterlab_chat is installed

[dlqqq]

Description

This PR introduces UI handling for document resets occurring on *.chat files provided by jupyterlab_chat. This may occur when an out-of-band change is made to a *.chat file or when the YDoc history is squashed to preserve server memory (a planned feature).

• Closes Handle reset of ychat documents in the frontend #160
• Closes Implement out of band logic for chat documents #107

Demo

Screen.Recording.2025-10-16.at.5.02.09.PM.mov

Technical details

• The document reset handling is implemented entirely in a single ychat plugin. This provides a custom YChat that provides a reset() method & resetSignal property for the Yjs Websocket provider to call & emit to when receiving a document reset close code (4xxx). This follows the strategy applied to our custom YFile and YNotebook classes.

• The ychat plugin listens to resetSignal on each YChat instance. When a reset is detected, the plugin finds the correct ChatWidget document widget containing the YChat instance which was reset, then clears its UI state before the subsequent SS1 + SS2 handshake is completed. This provides handling of chat document resets in the UI, without the need to fork the plugin providing the ChatWidget class.

  • The original ChatWidget plugin in jupyterlab_chat is 250 lines long and requires 14 other plugins.

• This PR adds NPM dependencies on @jupyter/chat and jupyterlab-chat. It also adds the correct JupyterLab module federation configuration to ensure that tokens and classes imported are the same instances as the ones provided with the jupyterlab_chat package.

  • This does NOT add jupyterlab_chat as a required dependency. The custom ychat plugin is only loaded if jupyterlab_chat is installed.

[ellisonbg]

I have done local testing and everything worked as is. I left some relative minor comments for changes and follow up issues. Marking as changes needed, but they should be relatively minor.

[dlqqq]

Implemented requested changes & verified that the branch still works as expected in handling document resets on chat files. 👍

[Zsailer]

This looks great, @dlqqq!

Looks like you addressed all of @ellisonbg's comments. Approving to unblock merge.