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.

src/app/api/proxy-pools/vercel-deploy/route.js

@@ -20,14 +20,15 @@ export default async function handler(req) {
 
   const targetUrl = target.replace(/\\/$/, "") + relayPath;
 
-  const headers = new Headers(req.headers);
-  headers.delete("x-relay-target");
-  headers.delete("x-relay-path");
-  headers.delete("host");
+  const rawHeaders = {};
+  for (const [k, v] of req.headers.entries()) rawHeaders[k] = v;
+  delete rawHeaders["x-relay-target"];
+  delete rawHeaders["x-relay-path"];
+  delete rawHeaders["host"];
 
   const response = await fetch(targetUrl, {
     method: req.method,
-    headers,
+    headers: rawHeaders,
     body: req.method !== "GET" && req.method !== "HEAD" ? req.body : undefined,
     duplex: "half",
   });