Preserve request headers through the Vercel/Cloudflare/Deno relay.
- proxyFetch: normalize options.headers via Object.fromEntries before
spreading into the relay headers. Spreading a Headers instance yields
{} and silently dropped every entry (auth + content-type), which is why
the same pool worked on one path and failed on another.
- vercel relay: build the forwarded header object from req.headers.entries()
instead of new Headers(req.headers), avoiding edge-runtime normalization
of casing/duplicate keys that some providers reject.
