fix(eio): expose EventEmitter interface on WebSocket upgrade response

[nabsei]

The kind of change this PR does introduce

• a bug fix
• a new feature
• an update to the documentation
• a code change that improves performance
• other

Current behavior

Fixes #5072. During a WebSocket upgrade, engine.io applies middlewares against a WebSocketResponse object that only implements a small subset of http.ServerResponse (setHeader/getHeader/removeHeader/write/writeHead/end). It does not implement EventEmitter, so any middleware that calls res.on(...) — for example pino-http, which listens for "close" to know when the request finished — crashes with TypeError: res.on is not a function.

New behavior

WebSocketResponse now extends EventEmitter and emits "close" when the underlying socket closes, and "finish" when end() is called, mirroring the events a real http.ServerResponse emits. Added a regression test in test/middlewares.js that registers a res.on("close", ...) listener during a WebSocket upgrade and asserts it fires.

Other information (e.g. related issues)

Parts of this fix (root-cause analysis and implementation) were produced with the help of Claude Code — credited via the Co-authored-by trailer on the commit. I reviewed and understood the change before opening this PR.

Verification performed:

• Added unit test passes, along with the full existing middlewares.js/server.js/engine.io.js suite (175 passing, 2 pre-existing unrelated pending) — no regressions.
• Reproduced the exact bug report with the real pino-http package against an unpatched build (crashes with res.on is not a function), then confirmed the same script completes cleanly with this fix applied.
• prettier --check passes on the changed files.