Add button to show outline in Positron Notebook Editor action bar

[dhruvisompura]

Fixes #12839

Adds a Toggle Outline button to the Positron notebook editor's action bar. Clicking it opens and focuses the Outline view in the Explorer pane; clicking again collapses it. This makes the notebook table of contents more discoverable for users coming from JupyterHub/JupyterLab who don't expect to find it in a side pane.

The same toggle is reachable from the command palette by searching for Notebook: Toggle Outline (command ID is positronNotebook.toggleOutline).

Screenshots

Screen.Recording.2026-05-20.at.2.25.47.PM.mov

Release Notes

New Features

• Added a Toggle Outline button to the Positron Notebook Editor action bar (Add a button to open the Outline (TOC) in the notebook navigation bar #12839)

Bug Fixes

• N/A

Validation Steps

@:notebooks
@:positron-notebooks
@:outline

1. Open a Jupyter notebook (.ipynb) in the Positron notebook editor.
2. In the editor's action bar, click the new outline button (between the Markdown and Ask Assistant buttons).
3. Verify the Explorer pane opens (if hidden), the Outline section expands and receives focus, and shows the notebook's TOC entries.
4. Click the button again. Verify the Outline section collapses.
5. With the Outline section collapsed, click the button. Verify it expands and focuses again.
6. Switch the sidebar to a different view (e.g. Search), then click the button. Verify the sidebar switches back to Explorer with Outline expanded and focused.
7. Open the command palette and run Notebook: Toggle Outline. Verify the same toggle behavior.

[github-actions]

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

^{readme  valid tags}