Support pre-filling chat input via public command API

[MUFFANUJ]

Third-party extensions need a stable public API to open JupyterLite AI chat and pre-fill the prompt text (for example i was working on jupyterlab/plugin-playground#136, which requires pre-filling the chat with a prompt).
For now, we rely on tracker/model internals (IChatTracker -> widget.model.input) after opening chat.

Current behavior

@jupyterlite/ai:open-chat supports:

• area
• name
• provider

It does not support pre-filling input directly.

A possible solution could be:

To support one of these public APIs:

1. Extend @jupyterlite/ai:open-chat args with:

• input?: string
• focus?: boolean (default true)
• autoSend?: boolean (default false)

or

2. Add a dedicated command, e.g.:
   @jupyterlite/ai:open-chat-with-message

[brichet]

Currently @jupyterlite/ai:open-chat doesn't check if a chat is already opened with the same provider.

An other option could be to add a command like @jupyterlite/ai:open-or-reveal-chat, that would:

• look for the chat in the tracker
  • if there is no chat call @jupyterlite/ai:open-chat with the expected args
  • if there is one, maybe call @jupyterlite/ai:move-chat if the area changed
• activate the widget
• fill the input if an args is provided

That would probably require to update @jupyterlite/ai:open-chat and @jupyterlite/ai:move-chat to return a widget instead of a boolean, to ease the input model retrieval after opening/reveling.