Integration testing for remote resources

There should be integration tests verifying, that, e.g., correct HTTP headers are sent when retrieving remote resources. Crucially, these should not require network access to use (which would, e.g., break the nix build, but could also lead to spurious failures), so this requires some kind of embedded web server.

Start by locating the Rust code that retrieves remote resources and the existing integration-test setup. Determine how an embedded web server can exercise HTTP headers without network access; done means the relevant headers are verified in integration tests that work during a network-isolated build.

Written by the indexing model from the issue text.