Add modern Node.js end-to-end test suite and CI

[Copilot]

The repo only had legacy Perl Test::Nginx::Socket tests and no CI. This adds a self-contained, modern e2e suite that builds nginx with the module and drives the public WebSocket endpoint with a standards-compliant client, runnable locally and in GitHub Actions.

Changes

• Test harness (e2e/harness.mjs) — spins up a TCP echo upstream (stand-in for a websockify/VNC backend) and an nginx instance built with the module on ephemeral ports, with config validation and automatic teardown.
• Tests (e2e/test/websockify.test.mjs) — Node's built-in node:test runner + ws, covering: binary round-trip, base64 decode/re-encode, subprotocol preference (binary over base64), large (1 MiB) streaming, rejection of unsupported subprotocols, and dead-upstream teardown.
• Build helper (e2e/build-nginx.sh) — compiles a configurable nginx version (NGINX_VERSION, nginx.org with GitHub mirror fallback) with the module added; prints the binary path.
• CI (.github/workflows/e2e.yml) — runs the suite on push/PR across nginx 1.24.0 / 1.25.3 / 1.27.3, with permissions: contents: read.
• Docs — e2e/README.md plus a Testing section in README.md.

cd e2e
npm install
export NGINX_BIN="$(./build-nginx.sh)"   # or point at an existing build
npm test

Note on observed behavior

With an unreachable upstream the module completes the WebSocket handshake (101) and then closes abnormally (1006) rather than returning 502. The dead-upstream test asserts against this actual behavior; worth confirming it is intended.