Add Jupyter-style keybindings for undo, redo, and cell commands

[nstrayer]

Addresses #3014.

This PR implements Jupyter-style keyboard shortcuts for cell-level undo, redo, copy, cut, and paste operations in Positron Notebooks. These shortcuts match standard Jupyter behavior and work when in command mode (notebook focused but not editing a cell).

Followup to #2357.

Implemented shortcuts:

• Z - Undo last cell operation
• Shift+Z - Redo cell operation
• C - Copy selected cell(s)
• X - Cut selected cell(s)
• V - Paste cell(s) below
• Shift+V - Paste cell(s) above

These shortcuts only activate when the notebook container is focused but no cell editor is focused (command mode), ensuring they don't interfere with normal text editing.

Release Notes

New Features

• N/A

Bug Fixes

• N/A

QA Notes

@:notebooks

1. Open a notebook in Positron
2. Add several cells with content
3. Press Esc to enter command mode (cell should have blue border, not green)
4. Test each shortcut:
   • Select a cell and press C to copy
   • Press V to paste below, or Shift+V to paste above
   • Press X to cut a cell
   • Press Z to undo the operation
   • Press Shift+Z to redo
5. Verify shortcuts only work in command mode (when no cell editor is focused)
6. Press Enter to enter edit mode and verify C, X, V, Z work as normal text editing keys

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:notebooks

^{readme  valid tags}

[Copilot]

Pull Request Overview

This PR adds Jupyter-style keyboard shortcuts for notebook cell operations, replacing traditional OS-specific shortcuts (Cmd/Ctrl+C/X/V/Z) with single-key shortcuts (C/X/V/Z) that only work in command mode when no cell editor is focused.

Key Changes

• Adds single-key shortcuts (Z, Shift+Z, C, X, V, Shift+V) for undo/redo and cell copy/cut/paste operations
• Implements command mode detection using POSITRON_NOTEBOOK_EDITOR_CONTAINER_FOCUSED and negated POSITRON_NOTEBOOK_CELL_EDITOR_FOCUSED context keys
• Removes OS-specific keybinding modifiers (CtrlCmd) from cell operation commands

[Copilot]

Pull Request Overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

[dhruvisompura]

These changes worked well for me! Looks good once the conflict is resolved.

I did want to ask about this comment in the PR description:

Press Esc to enter command mode (cell should have blue border, not green)

I don't see a green border around the cells at any point, is that supposed to be the case?

One thing I did notice while testing these keybindings is that its a bit annoying to do a sequence of actions like copy + paste because of how we automatically enter edit mode when we add a new cell to the notebook. The sequence to copy and paste a cell ends up being C, Escape, V.

I'm wondering if we should revisit that notion? Doesn't have to be in this PR!

[seeM]

Looks great! I agree with @dhruvisompura it shouldn't enter edit mode when new cells are added. Jupyter Lab behaves the way we're suggesting as well

[nstrayer]

@seeM and @dhruvisompura Re: the entering-edit-mode annoyance #10120