Automated tests for the VS Code extension

[Weidav]

Description

The vscode-pyright extension currently has no automated tests, only for pyright-internals. The interpreter-resolution logic in extension.ts is needs manuall testing.

This came up in #1860, which adds support for the new Python Environments API alongside the classic ms-python.python API. That change is entirely untested in CI, and it's exactly the kind of logic that regresses easily and is annoying to test by hand.

Suggestion
A test setup for the extension so this and future interpreter-resolution changes can be covered automatically.

Integration tests via @vscode/test-electron that launch a real extension host, install the relevant extensions, switch interpreters, and assert basedpyright resolves against the expected environment.
I don't see the need for unit-tests.

I maintain a set of python test fixtures for uv-auto-venv — small Python projects with differing dependencies, so missing/failed imports surface immediately. These would work well as fixtures for the integration tests here (raised originally in #1860).

Open question
Should I only prepare the test-setup and let you write the tests? Or do I also write the test with the help of AI and you do the review?

[Weidav]

I'l only submit the setup and a blueprint-test. @DetachHead respondet to the question here:

#1860 (comment)

i appreciate you asking first before throwing a bunch of AI generated code my way. in my experience, LLMs seem to be very bad at writing tests. the tests they write often don't actually test anything at all.

with that in mind, and considering that i've recently added a policy prohibiting AI generated code, i would much prefer this option:

I could only prepare the setup and add a test or two as a blueprint and let you write the rest of the tests

thanks