Hide irrelevant upstream notebook commands in Positron notebooks

[nstrayer]

Addresses #10481.

Hides upstream VS Code notebook commands (developer troubleshooting, layout configuration, sticky scroll, etc.) from the command palette when the Positron notebook editor is active. These commands target the upstream notebook implementation and don't apply to Positron notebooks.

Introduces a shared POSITRON_NOTEBOOK_IS_NOT_ACTIVE_EDITOR context key expression used as a precondition (for f1: true commands) or when clause (for explicit MenuId.CommandPalette entries) to gate visibility. Also replaces a hard-coded editor ID string in ghost cell actions with the POSITRON_NOTEBOOK_EDITOR_ID constant.

Before changes (note developer commands etc.)

After changes

Release Notes

New Features

• N/A

Bug Fixes

• Hide irrelevant upstream notebook commands from the command palette when using the Positron notebook editor (Positron Notebooks: Only show available commands in the command palette for the Positron Notebook Editor #10481)

QA Notes

@:notebooks @:positron-notebooks

1. Open a .ipynb file in the Positron notebook editor
2. Open the command palette (Cmd+Shift+P)
3. Search for "notebook" -- verify the following commands are not listed:
   • Customize Notebook Layout
   • Toggle Notebook Sticky Scroll
   • Toggle Notebook Layout Troubleshoot
   • Inspect Notebook Layout
   • Clear Notebook Editor Type Cache
   • Clear Notebook Kernels MRU Cache
   • Toggle Notebook Clipboard Troubleshooting
   • Toggle Notebook View Zones
   • Add Cell Output to Chat
4. Open a .ipynb in the standard VS Code notebook editor (disable Positron notebook) and verify those commands do appear

[github-actions]

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

^{readme  valid tags}

[cindyytong]

This looks great!