Function sanitize_uri 

pub(crate) fn sanitize_uri(uri: &str) -> String

Sanitizes a single URI string by ensuring it has a proper scheme.

Mirrors the logic from the iOS fixURLIfNeeded() method:

1. If the URI is already a valid URL, return as-is.
2. If prepending http:// yields a URL whose host is an IPv4 address, use http://.
3. If the URI doesn’t already start with http, prepend https://.
4. Otherwise, return as-is.