stack (native runtime): WebSocket upgrade on a reused keep-alive connection gets 101 but is never relayed — Realtime never subscribes #7429

Summary

With the experimental stack engine ([experimental] stack = true, supabase start --runtime native, CLI 2.122.0, macOS arm64), a Realtime WebSocket that a Node.js process opens after it has made ordinary HTTP requests to the same API port never goes live. The gateway answers the upgrade 101, but no frame is relayed, so channel.subscribe() never reaches SUBSCRIBED.

Node's built-in WebSocket (undici) upgrades through the process's shared HTTP connection pool, so it reuses an idle kept-alive connection to the gateway. A fresh process, with no requests before the WebSocket, subscribes in ~12 ms. The Docker runtime's Kong gateway did not have this problem.

Reproduction (Node 24.15, @supabase/supabase-js 2.116.0)

import { createClient } from "@supabase/supabase-js";
const c = createClient(API_URL, SECRET_KEY, { auth: { persistSession: false } });
// a few ordinary requests to the same gateway, on kept-alive connections
for (let i = 0; i < 6; i++) await Promise.all([1, 2, 3].map(() => c.from("profiles").select("id").limit(1)));
c.channel("job:x", { config: { private: true } }).subscribe((s) => console.log(s)); // never SUBSCRIBED

• Without the for loop: SUBSCRIBED in ~12 ms.
• With it: nothing for 10 s+. The gateway log shows GET /realtime/v1/websocket?... HTTP/1.1" 101; Realtime logs no join and no error; realtime-js's logger shows push phx_join but never transport connected.
• Giving the WebSocket its own connections works around it (new WebSocket(url, { dispatcher: new Agent() }) from undici): SUBSCRIBED in ~6 ms.

Expected

A WebSocket upgrade on a reused HTTP/1.1 keep-alive connection is relayed like one on a new connection (as with the Docker runtime's gateway).