fix(request): use PassThrough as socket for IncomingMessage

[alex-all3dp]

Problem

@codegenie/serverless-express >= 4.16.0 with Express 5 throws TypeError: ee.on is not a function whenever a request reaches Express's finalhandler (e.g. unmatched routes). Reported in #698; still present in v5.0.0.

Stack from production:

TypeError: ee.on is not a function
    at first (ee-first/index.js:43:10)
    at onSocket (on-finished/index.js:115:16)
    at attachFinishedListener (on-finished/index.js:120:5)
    ...
    at send (finalhandler/index.js:291:3)

Root cause

ServerlessRequest (src/request.js) extends http.IncomingMessage and passes a plain object as the socket. When Express 5's finalhandler reaches an unmatched route and the request body wasn't consumed (typical for GETs), it calls onFinished(req), which uses ee-first to attach error and close listeners on req.socket. The plain object has no .on, so it throws.

Fix

Use a PassThrough stream as the socket. PassThrough inherits the EventEmitter interface via Duplex/Stream, so .on, .once, .emit, and .removeListener are all present. The end/destroy Function.prototype stubs are dropped since PassThrough provides real implementations.

This is the same fix that landed upstream in dougmoscrop/serverless-http#306 (merged Sep 2025), the library this file was forked from per the attribution comment at the top.

Closes #698