[Bug]: proxy forwards headers named by Connection

Description

ProxyUtil.headers() removes the fixed hop-by-hop header set, but it leaves extension headers named by the incoming Connection header.

For example, with Connection: x-hop, the Connection header is removed while X-Hop: must-not-forward is still sent upstream. RFC 9110 §7.6.1 requires an intermediary to remove every field named by Connection before forwarding. Both HTTP proxy call sites use this helper.

Steps to reproduce

1. From packages/opencode, run:

   bun -e 'import { ProxyUtil } from "./src/server/proxy-util.ts";
   console.log(Object.fromEntries(ProxyUtil.headers({
     connection: "x-hop",
     "x-hop": "must-not-forward",
     "x-end": "keep"
   })))'
   

2. Observe that the result still contains x-hop:

   { "x-end": "keep", "x-hop": "must-not-forward" }
   

Expected: connection and x-hop are absent, while x-end remains.

Operating System

All platforms