fix: encodeViaTurboStream leaks memory via unremoved AbortSignal listener

[luchsamapparat]

encodeViaTurboStream registers a clearStreamTimeout listener on requestSignal but never removes it. Even though #14735 correctly added onComplete: clearStreamTimeout to clear the timer when encoding finishes, the listener itself remains attached to the AbortSignal for the lifetime of the Request object.

Because the clearStreamTimeout closure captures timeoutId, the Timeout object (and everything it transitively retains) cannot be GC'd until the Request is eventually collected. Under sustained request load this causes unbounded accumulation of live Timeout objects, like in this heap snapshot:

In our case, we use AsyncLocalStorage to scope per-request state. Because of this leak, each Timeout retains its full AsyncContextFrame and with it the entire ALS store associated with the request.

The above screenshot is from running 100 E2E tests against our application.

This is a heap snapshot after the same 100 E2E tests after applying the fix:

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[rossipedia]

@mcollina You originally added the timeout in #14735, is this accurate?

Either way I don't see any harm here in being thorough

[jacob-ebey]

I've re-targeted this at dev and forwarded the request abort signal through at #14900