bug: aborted signal not handled properly by libp2p router

See https://github.com/ipfs/service-worker-gateway/pull/681#issuecomment-2816001952.}

I don't have a good repro usecase besides some code changes in https://github.com/ipfs/service-worker-gateway/pull/681 and manual testing.

It's difficult to set up tests with all the things enabled that I need..

It's somewhere in the combination of `ipfs-unixfs-exporter` and `@helia/routers` when bitswap is enabled.

I was able to get this fixed in verified-fetch with:

```js
import { abortableSource } from 'abortable-iterator'

// ...

const iterator = options?.signal != null
? abortableSource(exporterWalk(path, blockstore, options), options.signal)
: exporterWalk(path, blockstore, options)
for await (const entry of iterator) {

// ...
```