node:http: expose IncomingMessage.setEncoding for decoded data chunks #3416

Summary

Client-side http.IncomingMessage responses should expose setEncoding(encoding) and then emit decoded string chunks from data listeners. Perry currently does not expose the method as a function, so user code cannot request Node-compatible decoded response chunks.

Node Behavior

Minimal repro:

import http from "node:http";
import { Buffer } from "node:buffer";

const server = http.createServer((_req, res) => {
  res.writeHead(200, { "Content-Type": "text/plain; charset=utf-8" });
  res.end(Buffer.from("héllo", "utf8"));
});

server.listen(0, () => {
  const addr = server.address();
  const port = typeof addr === "object" && addr !== null ? addr.port : 0;
  http.get({ host: "127.0.0.1", port, path: "/" }, (res: any) => {
    console.log("setEncoding typeof:", typeof res.setEncoding);
    res.setEncoding("utf8");
    res.on("data", (chunk: any) => {
      console.log("chunk:", typeof chunk, Buffer.isBuffer(chunk), String(chunk));
    });
    res.on("end", () => server.close());
  });
});

Node v25.9.0 observes:

setEncoding typeof: function
chunk: string false héllo

Perry Behavior

A focused parity probe in test-files/tmp_http_incoming_set_encoding.ts fails before the chunk-shape assertion because Perry reports the method as a non-function:

Node.js: setEncoding typeof: function
Perry:   setEncoding typeof: number

Report: test-parity/reports/parity_report_20260530_103216.json.

Source evidence on origin/main: crates/perry-ext-http/src/lib.rs always delivers client response data listeners as a single Buffer chunk and has a TODO noting that res.setEncoding("utf8") should produce string chunks, but Perry does not yet track a per-response encoding flag.

Suggested PR Cut

Batch this with related issues in:
node:http: IncomingMessage body stream semantics

Good batch candidates:

• node:http: complete IncomingMessage metadata and timeout surface #2540
• node:http: request 'data' chunks are strings not Buffers → Buffer.concat throws #3273

Do not batch with:

• top-level node:http helper exports
• ClientRequest header/socket-control work
• byte-level response framing/header output diffs

Acceptance

• parity/regression test proves client response IncomingMessage.setEncoding("utf8") is a function and returns/behaves like Node
• data listeners receive decoded string chunks after an encoding is set
• default no-encoding data listeners continue to receive Buffer chunks
• implementation matches Node observable behavior
• related known-failure/docs/manifest entries updated if touched