Export Jupyter notebooks to percent script format (Python & R)

[seeM]

Toward #9791.

This PR adds functionality to export Jupyter notebooks to other file formats. It's designed so that third party extensions can contribute their own notebook exporters. I packaged this in a new extension (rather than the positron-notebooks-helper extension) since it has a public-facing API.

Here is the corresponding PR setting up an exporter in the Quarto extension: quarto-dev/quarto#999.

Demo:

Screen.Recording.2026-06-10.at.17.29.06.mov

Release Notes

New Features

• Export Jupyter notebooks to Quarto (.qmd), Python, and R (Positron Notebooks: Export to .qmd or .py/.R #9791)

Bug Fixes

• N/A

Validation Steps

@positron-notebooks @:web

1. Open a Python or R Jupyter notebook
2. Click the "..." menu in the notebook toolbar
3. Click "Export..."
4. Choose Python or R
5. Verify that the exported file is opened in a new, untitled editor tab

[github-actions]

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

^{readme  valid tags}

[github-actions]

PETE's assessment 🧪

Verdict: 🟢 Adequate -- The PR ships both extension-host tests for the command/registration flow and unit-style tests for the percent-script conversion, covering the new behavior at the cheapest viable levels.

What changed

• New positron-notebook-export extension contributing a notebook.export command, a public registerNotebookExporter API, and builtin Python/R percent-script exporters (#9791).
• Core conversion logic lives in percentNotebookExporter.ts (notebookToPercentScript); command/picker orchestration in notebookExportCommand.ts; language detection in util/notebook.ts.
• Plus build wiring (dirs.ts, gulpfile.extensions.ts, .vscode-test.js) and a one-line positron-notebooks/tsconfig.json cleanup.

Tests in this PR

• Unit (Vitest/Mocha) ✅ (percentNotebookExporter.test.ts -- code/markdown/raw/multi-cell/custom-prefix cases)
• Extension host ✅ (extension.test.ts -- empty picker, language filtering, export invocation, dispose/unregister)
• E2E (Playwright) ✅ (not warranted -- export logic is unit-covered; picker flow is stub-covered at ext-host level)

Existing coverage

The two new test files are the primary coverage and exercise the changed behavior directly: percentNotebookExporter.test.ts asserts exact output for every cell type and a custom comment prefix; extension.test.ts drives the real notebook.export command through the registration API, language filtering, selection, and disposal. No existing tests were relied upon.

Suggested additions

None required. Minor optional gap: getNotebookLanguage (util/notebook.ts) is only exercised via the metadata branch (the test notebook declares language_info.name: "test" and has no cells). The cell-language fallback (lines 26-31) and the undefined return are untested -- a small percentNotebookExporter-style or direct unit test of getNotebookLanguage covering a metadata-less notebook would close that.

Deployment note (optional)

The PR body tags @:web, which is appropriate: export() opens an untitled document via workspace.openTextDocument/showTextDocument, and the picker uses ThemeIcon.File + a file resourceUri -- both behave differently or are constrained in the web build (no native file backing). The ext-host tests stub showQuickPick and the exporter's export, so the real untitled-editor round-trip isn't exercised on web at PR time. The @:web tag is in place, but no @:positron-notebooks- or @:web-tagged e2e currently exercises notebook export specifically; consider a follow-up e2e (per the validation steps in the PR body) once the feature stabilizes.

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[seeM]

The failing unit tests seem to be an issue on main, since I'm seeing the same failures in other PRs.