ws 8.18.0: Prove that WebSocket.Server's headers event allows appending custom HTTP headers to the upgrade response

The `search_known_solution` query for `WebSocket.Server` and its `headers` event returned no matches.

This sample pins `ws` at version 8.18.0, the current stable major branch.
The exact observed behaviour is that the `headers` event emitted by `WebSocket.Server` provides an array of HTTP headers that will be sent in the upgrade response. Mutating this array directly, such as by pushing `'X-Custom-Header: yes'`, successfully injects the header into the response sent to the client, which can be verified in the client's `upgrade` event payload.

{"case":{"caseId":"case:sha256:9228cc375b361966cf13821a0e25d65c73b9c62c4499d0721ecd335b16b5cb6c","contract":["The headers array passed to the 'headers' event listener can be mutated to send custom headers back to the client during the WebSocket handshake.","A client can read these custom headers during its 'upgrade' event."],"goal":"Prove that WebSocket.Server's headers event allows appending custom HTTP headers to the upgrade response","kind":"HOW","packages":["pkg:npm/ws@8.18.0"],"schemaVersion":1,"symbols":["WebSocketServer"]},"contractCommand":["node","test/contract.mjs"],"environment":{"arch":"x64","ecosystem":"npm","executionContext":"node","language":"node","os":"linux","packageManager":"npm","runtime":"node","schemaVersion":1},"license":"MIT-0","packages":["pkg:npm/ws@8.18.0"],"schemaVersion":1,"symbols":["WebSocketServer"],"verifierAdapter":"node-typescript@1"}

{
  "name": "npm-ws-version-depth",
  "version": "1.0.0",
  "lockfileVersion": 3,
  "requires": true,
  "packages": {
    "": {
      "name": "npm-ws-version-depth",
      "version": "1.0.0",
      "license": "ISC",
      "dependencies": {
        "ws": "8.18.0"
      }
    },
    "node_modules/ws": {
      "version": "8.18.0",
      "resolved": "https://registry.npmjs.org/ws/-/ws-8.18.0.tgz",
      "integrity": "sha512-8VbfWfHLbbwu3+N6OKsOMpBdT4kXPDDB9cJk2bJ6mh9ucxdlnNvH1e+roYkKmN9Nxw2yjz7VzeO9oOz2zJ04Pw==",
      "license": "MIT",
      "engines": {
        "node": ">=10.0.0"
      },
      "peerDependencies": {
        "bufferutil": "^4.0.1",
        "utf-8-validate": ">=5.0.2"
      },
      "peerDependenciesMeta": {
        "bufferutil": {
          "optional": true
        },
        "utf-8-validate": {
          "optional": true
        }
      }
    }
  }
}

{
  "name": "npm-ws-version-depth",
  "version": "1.0.0",
  "private": true,
  "type": "module",
  "dependencies": {
    "ws": "8.18.0"
  },
  "description": "",
  "main": "index.js",
  "directories": {
    "test": "test"
  },
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC"
}

import { WebSocketServer, WebSocket } from 'ws';
import assert from 'node:assert';

const wss = new WebSocketServer({ port: 0 });

wss.on('headers', (headers, request) => {
  headers.push('X-Custom-Header: yes');
});

wss.on('listening', () => {
  const port = wss.address().port;
  const ws = new WebSocket(`ws://127.0.0.1:${port}`);
  
  ws.on('upgrade', (res) => {
    try {
      assert.strictEqual(res.headers['x-custom-header'], 'yes');
      process.exit(0);
    } catch (err) {
      console.error(err);
      process.exit(1);
    }
  });
});

import { WebSocketServer, WebSocket } from 'ws';
import assert from 'node:assert';

const wss = new WebSocketServer({ port: 0 });
wss.on('headers', (headers, request) => {
  headers.push('X-Custom-Header: yes');
});

wss.on('listening', () => {
  const port = wss.address().port;
  const ws = new WebSocket(`ws://127.0.0.1:${port}`);
  
  ws.on('upgrade', (res) => {
    try {
      assert.strictEqual(res.headers['x-custom-header'], 'yes');
      console.log('Contract passed');
      ws.close();
      wss.close();
    } catch (err) {
      console.error(err);
      process.exit(1);
    }
  });
});