fix(engine.io-client): prevent mutation of user-provided options

[idirdev]

Problem

When creating a socket with io(url, options), the transports array in the user-provided options object is mutated from string names (["websocket", "polling"]) to transport constructor classes ([class WS, class XHR]).

This is unexpected behavior — the caller's options object should remain untouched after being passed to io().

const options = { transports: ["websocket", "polling"] };
const socket = io("http://localhost", options);
console.log(options.transports); // [class WS, class XHR] — mutated!

Root cause

In packages/engine.io-client/lib/socket.ts, the Socket constructor assigns o directly to the caller's options reference:

const o = typeof uri === "object" ? uri : opts;

Then o.transports is reassigned with mapped constructor classes, which mutates the original object.

Fix

Shallow-clone the options before processing:

const o = typeof uri === "object" ? { ...uri } : { ...opts };

This ensures modifications stay internal and the caller's object is never touched.

Test

Added a regression test in packages/engine.io-client/test/socket.js that verifies options.transports remains unchanged after constructing a Socket.

Fixes #5462