Add unit test coverage for registry that requires basic HTTP authentication

[zanieb]

We're missing coverage for this and it's fairly critical.

We'll need to host an index somewhere that we can expose credentials to.

e.g. to prevent regressions like #2444

[zanieb]

A NGINX proxy with HTTP basic authentication in progress at astral-sh/pypi-proxy#1

[vlad-ivanov-name]

Not sure if this is useful but a few weeks ago I implemented a "simple" pip server for integration tests in pixi:

https://github.com/vlad-ivanov-name/pixi/blob/f52c9ac1772cb623f257cd222e303480bcf76940/tests/common/pypi_server.rs

pixi has since switched to uv as backend so the associated PR was obsolete and was closed, but perhaps the code could be useful here. It creates a local server with Axum and creates wheels on the fly -- all from rust, without needing external processes, containers, etc. Authentication would be really easy to add there via axum auth middleware.

[zanieb]

Awesome thanks! It's on my wishlist to have a local server for the test suite. I'll take a look at that.