Add external-link button to the Packages pane

[bricestacey]

Summary

Adds an external-link button to each Packages pane row. When a package
advertises a website, the row shows a button (beside the help button) that
opens it in the user's external browser.

The URL comes from the language runtime via the existing getPackages call
-- no third-party API, so it works offline. A new optional url is added to
ILanguageRuntimePackage (and the public LanguageRuntimePackage). The
Python kernel picks the single best URL from a distribution's Project-URL
metadata (homepage > repository > documentation, with the legacy Home-page
header as a homepage fallback). The frontend validates the scheme (http/https
only) before rendering the button, so a malformed or non-web scheme from a
runtime can never reach the opener.

R support comes from a companion ark PR (posit-dev/ark#1261); the R button
lights up once that lands and the ark submodule is bumped in Positron.

See #13890

Screenshots

Release Notes

New Features

• Added an external-link button to Packages pane rows that opens a package's website (Add external link action button to the Packages pane #13890)

Bug Fixes

• N/A

Validation Steps

@:packages-pane

1. Start a Python session and open the Packages pane.
2. Filter to a package that publishes a homepage (e.g. numpy, pandas, requests).
3. Confirm an external-link button appears beside the help button, and clicking it opens the package's website in your browser.
4. Confirm a package with no URL metadata shows no link button.

[github-actions]

E2E Tests 🚀
This PR will run tests tagged with: @:critical @:packages-pane

^{readme  valid tags}

[github-actions]

PETE's assessment 🧪

Verdict: 🟢 Adequate -- the URL-selection logic carries thorough unit coverage and the new frontend button has e2e render coverage, matching the cheapest viable level for each change.

What changed

• Python kernel learns to pick a single best package URL (_best_package_url in ui.py): ranks homepage > repository > documentation > other, with the legacy Home-page header as a homepage fallback; _get_packages_installed attaches the result as an optional url.
• A new optional url is threaded through the public LanguageRuntimePackage (positron.d.ts) and internal ILanguageRuntimePackage (runtimeSessionService.ts).
• The Packages pane (listPackages.tsx + .css) renders an external-link button per row, gated on matchesSomeScheme(url, http, https) so non-web schemes can't reach the opener.

Tests in this PR

• Unit (Vitest/Mocha) ✅ (Python test_ui.py covers the URL-ranking logic; frontend gate is a thin wrapper over a core util -- see below)
• Extension host ✅ (not applicable -- no activated-extension behavior changed)
• E2E (Playwright) ✅ (added to packages-pane.test.ts, new urlButton page-object locator)

Existing coverage

• extensions/positron-python/.../tests/test_ui.py adds test_best_package_url with 8 parametrized cases pinning the exact ranking contract (homepage-beats-repository, normalized labels, legacy Home-page, project-url-beats-legacy, unrecognized-fallback, and the None case). This is the substantive new logic and it's well covered at the cheapest level.
• test/e2e/tests/packages-pane/packages-pane.test.ts adds a URL button test asserting the link button is visible for numpy (which publishes a homepage), exercising the full kernel -> url field -> scheme-gate -> rendered button path. Reasonable to verify at e2e since it spans the runtime + workbench render.
• The frontend hasValidUrl gate is a single call to the already-tested core matchesSomeScheme; the ListPackages component has no existing vitest harness, so a dedicated unit test would be low-value relative to setup cost. The e2e covers the positive render.

Suggested additions

None. Optionally, the no-URL negative branch ("a package with no URL shows no button," per the PR's own validation steps) is asserted only on the Python side (_best_package_url -> None), not in the e2e; adding a toHaveCount(0) assertion for a URL-less package would round out the frontend gate, but the existing coverage is sufficient.

Deployment note (optional)

The new e2e test is tagged @:web (via the suite-level tags.WEB), so the link button renders in the web build too -- good, since openerService.open(..., { openExternal: true }) behaves differently across desktop and web. No Windows-specific concern here (the change touches no path/process/pty hotspots).

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PETE (Positron Extreme Test Experiment) - LLM-based test-coverage advisor, in pilot. Triggers on PR open and on /recheck-tests comments. Wrong verdict? Comment /recheck-tests (or /rePETE) on this PR to re-run. Please share feedback on how PETE performed here.

[austin3dickey]

This looks awesome, Brice! It's so cool to see this pane's progress.