fix: support rack-proxy 1.0 (close #622)

[ElMassimo]

Description 📖

This pull request makes ViteRuby::DevServerProxy compatible with rack-proxy 1.0, so proxied asset requests during development no longer fail with 502 Bad Gateway. Closes #622.

Background 📜

This was happening because #619 relaxed the dependency to rack-proxy >= 0.6.1, which lets bundler resolve rack-proxy 1.0. rack-proxy 1.0 refuses to derive a backend from the client-controlled Host header unless you explicitly opt in (an SSRF/open-proxy guard). DevServerProxy selected the Vite backend precisely by rewriting the Host header, so on rack-proxy 1.0 every proxied request returned [502, {}, []]:

rack-proxy: refusing Host-derived backend "localhost" (no :backend configured; pass allow_dynamic_backend: true to opt in)

The Fix 🔨

By changing forward_to_vite_dev_server to set an explicit, config-derived backend on the request env rather than relying on the Host header:

env["rack.backend"] = URI(config.origin)