fix(client): remove abort listener on success in fetchWithTimeout

[LeSingh1]

Fixes #1811.

fetchWithTimeout attaches an abort listener to the caller's signal that forwards to its internal controller. The listener is added with { once: true }, so it self-removes when the signal actually fires, but it is never removed when the request completes successfully.

On Node this is harmless because AbortSignal.timeout() keeps its timer unref'd regardless of listeners. On Deno, attaching a listener to a signal returned by AbortSignal.timeout() refs the underlying timer. The orphaned listener keeps that timer alive for the full timeout duration, so a request that completes in 500ms blocks process exit for the remainder of a caller's 30s timeout.

The fix removes the listener in the existing finally block alongside clearTimeout. The abort reference is already hoisted into the enclosing scope, so no other changes are needed.

Repro (from the linked issue)

import OpenAI from "openai";

const client = new OpenAI();
const signal = AbortSignal.timeout(30000);

const response = await client.chat.completions.create(
  { model: "gpt-4o", messages: [{ role: "user", content: "Say hi" }], max_tokens: 10 },
  { signal },
);

console.log("Result:", response.choices[0].message.content);
// Before: Deno hangs ~30s after logging. After: exits immediately.

Tests

Added a regression test in tests/index.test.ts that spies on addEventListener / removeEventListener of a caller-supplied AbortSignal and asserts the same listener reference is both added and removed once the request resolves. Full tests/index.test.ts suite (61 tests) passes against the mock server.