fix: remove abort event listener from caller signal after request completes

[xodn348]

Summary

Fixes #1811.

When a caller passes a signal to fetchWithTimeout, an abort listener is added on that signal to forward abort events to the internal AbortController. With { once: true }, the listener cleans itself up when the signal fires — but if the request completes successfully (without the signal aborting), the listener is never removed.

On Deno, AbortSignal.timeout() refs its underlying timer whenever a listener is attached. The orphaned listener keeps the timer alive for the full timeout duration (e.g. 30 s), preventing the process from exiting even after the request returned in ~500 ms.

Fix: call signal.removeEventListener('abort', abort) in the finally block, alongside the existing clearTimeout(timeout). This mirrors the symmetric cleanup already done for the setTimeout.

   } finally {
     clearTimeout(timeout);
+    if (signal) signal.removeEventListener('abort', abort);
   }

The abort variable is already a named reference (created by this._makeAbort(controller)), so it can be passed to both addEventListener and removeEventListener without any other changes.

Test plan

• Added a unit test (removes abort listener from caller signal after successful request) that spies on addEventListener/removeEventListener of the caller's signal and asserts:
  • the listener was added with { once: true }
  • removeEventListener was called with the same function reference after a successful fetch
• All existing tests in tests/index.test.ts continue to pass (54/54)