reject redirects to non-http(s) schemes in PipSession

[Arawoof06]

What does this PR do?

pip mounts a LocalFSAdapter for file:// URLs on the same session it uses for the index and for downloads, and requests follows a 3xx Location into any scheme the session has an adapter for. A remote server can therefore redirect a request to a file:// URL and have pip read a local path through LocalFSAdapter.send (on Windows a UNC target reaches an SMB share). get_redirect_target now refuses a redirect whose scheme is not http/https; relative targets keep the current scheme and direct file:// requests are unaffected.

PR Checklist:

• I agree to follow the PSF Code of Conduct.
• I have read and have followed the CONTRIBUTING.md file.
• I have added a news file fragment (or this PR does not need one).
• I have read and followed the AI_POLICY.md file, and if any AI tools were used, I have disclosed it below.

I use an LLM lightly for research and to sanity-check things, but the code here is mine and I understand and take full responsibility for it.

[sepehr-rs]

Hi @Arawoof06, thanks for your contribution.
I haven't looked closely at the changes yet, but I noticed the AI policy checkbox has been left unchecked on several of your PRs. Is that intentional?

[Arawoof06]

Not intentional, just an oversight. Ticked it now, and updated the other PRs too. I use an LLM a little for research and sanity-checking, but the code is mine and I understand and stand behind it. Happy to walk through the redirect logic if you want to dig in.