Summary

This PR fixes the bug where the server was not considering the cells.structure.didOpen field to sync up the new content of the newly added cells.

The parameters corresponding to this request provides two fields to get the newly added cells:

1. cells.structure.array.cells: This is a list of NotebookCell which doesn't contain any cell content. The only useful information from this array is the cell kind and the cell document URI which we use to initialize the new cell in the index.
2. cells.structure.didOpen: This is a list of TextDocumentItem which corresponds to the newly added cells. This actually contains the text content and the version.

This wasn't a problem before because we initialize the cell with an empty string and this isn't a problem when someone just creates an empty cell. But, when someone copy-pastes a cell, the cell needs to be initialized with the content.

fixes: #12201

Test Plan

First, let's see the panic in action:

1. Press Esc to allow using the keyboard to perform cell actions (move around, copy, paste, etc.)
2. Copy the second cell with c key
3. Delete the second cell with dd key
4. Paste the copied cell with p key

You can see that the content isn't synced up because the unused-import for sys is still being highlighted but it's being used in the second cell. And, the hover isn't working either. Then, as I start editing the second cell, it panics.

Now, here's the preview of the fixed version: