Ensure notebook document event registration

[msujew]

What it does

Ok, this one is a bit difficult to describe, but I will try my best:

The Jupyter plugin uses a context key called jupyter.ispythonnotebook to determine whether to show the jupyter.openVariableView button.

This context key is set inside of the jupyter plugin, after encountering the metadata.kernelSpec property. This metadata property is set immediately when the notebook opens. This is done by performing a notebook metadata edit. This edit is performed correctly in the frontend notebook model, but since the necessary events aren't setup yet, the metadata edit isn't propagated back to the plugin host.

This change adjusts the order in which the service calls are happening to ensure that all events are properly setup before the notebook opening event is sent to the plugin host. This ensures that any edit operation on the frontend side is correctly propagated towards the plugin host.

How to test

Open a notebook editor for a .ipynb file and select a kernel. Reload the app and ensure that the Variables button is available in the notebook toolbar.

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