[MrSubidubi]

Supersedes #62653

Summary

This PR unifies the download handling we have in the http_client crate, because we were alternating between the ways we were downloading a lot, in one case we changed the way of downloading entirely if a hex digest was present or not.

With this in, we now handle this one way for all kinds of downloads, removing a lot of duplicated code and unifying the behavior here across download types.

Testing

Tested this manually.

Self-review

• I've reviewed my diff for quality, security, reliability, and performance.
• UI changes follow the checklist.
• Tests cover the new or changed behavior.

Release Notes:

• Improved reliability of language server and agent downloads by verifying checksums consistently, rejecting failed HTTP responses, and improving cleanup of partial downloads.