fix(proxy-pools): lossless header forwarding for relay pools

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.

open-sse/utils/proxyFetch.js

@@ -298,8 +298,11 @@ export async function proxyAwareFetch(url, options = {}, proxyOptions = null) {
   const vercelRelayUrl = normalizeString(proxyOptions?.vercelRelayUrl);
   if (vercelRelayUrl) {
     const parsed = new URL(targetUrl);
+    const baseHeaders = options.headers instanceof Headers
+      ? Object.fromEntries(options.headers.entries())
+      : { ...(options.headers || {}) };
     const relayHeaders = {
-      ...options.headers,
+      ...baseHeaders,
       "x-relay-target": `${parsed.protocol}//${parsed.host}`,
       "x-relay-path": `${parsed.pathname}${parsed.search}`,
     };