Add parametrized package manager roundtrip integration test

[edvilme]

Summary

Adds a parametrized integration test that verifies every discovered package manager performs a full package lifecycle correctly, using only the public extension API.

Roundtrip (per manager, all via the public API)

1. getPackages — baseline (test package absent)
2. managePackages({ install }) — install the test package
3. refreshPackages + getPackages — test package now present
4. direct dependencies — derived from Package.isTransitive
5. managePackages({ uninstall }) — uninstall
6. getPackages — test package absent again

Design

• No custom harness. The test drives only the exported PythonEnvironmentApi.
• Parametrized by discovery. Environments are grouped by envId.managerId (one representative per manager, preferring venv-like/modifiable envs), so any future package manager is exercised automatically with no per-manager code.
• Failures are aggregated and reported per manager; managers without a usable package manager, or where the test package is already present, are skipped gracefully.
• Best-effort finally cleanup guarantees the environment is never left dirty.

Known limitation

• Available versions is intentionally not exercised. getPackageAvailableVersions (and getDirectPackageNames) live only on the internal PackageManager interface and are not surfaced on the exported PythonEnvironmentApi, so they are unreachable from a public-API-only test. Direct (non-transitive) packages are still covered because Package.isTransitive is public.

Testing

• npm run compile-tests
• npm run lint (new file clean)
• Full roundtrip runs in the extension host via npm run integration-test (CI).