Bugfix - Fix WorkerEntrypoint use-after-free on subrequest cancellation

[jutaz]

WorkerEntrypoint::requestImpl is a coroutine whose KJ_DEFER at scope exit runs incomingRequest->drain(waitUntilTasks, ...). The DEFER is invoked either at natural scope exit (with this alive) or from the coroutine's destructor. On some subrequest paths the coroutine's destructor fires after the WorkerEntrypoint has already been freed by its owning Own<WorkerInterface>: the promise chain that owns the coroutine can outlive that Own via the response body's attached task.

The trigger observed in practice is a fire-and-forget env.QUEUE.send() followed by a 3xx response — the queue producer's loopback subrequest lands its WorkerEntrypoint::requestImpl coroutine in IoContext::tasks, and when the outer request unwinds that TaskSet the coroutine's DEFER reads this->waitUntilTasks from freed memory, gets a null TaskSet&, and faults inside TaskSet::add. Repro binaries either segfault (bazel test harness) or spin at 100% CPU indefinitely (standalone workerd / miniflare).

Bind waitUntilTasks to a coroutine-frame local so the DEFER's implicit [&] capture pins the reference against the coroutine frame instead of this. WorkerService::waitUntilTasks itself lives for the service's lifetime, so the underlying TaskSet is always valid.

The DEFER still reads other this->* members (failOpenService, proxyTask, loggedExceptionEarlier, abortController, cfBlobJson); those are latent UAFs on the same path but haven't been observed to crash — zeroed freed memory happens to steer the DEFER's control flow past them. A proper fix would tie the entrypoint's lifetime to the coroutine's; leaving a TODO(cleanup). Happy to refactor this to address the TODO as well.

Includes a wd_test regression that exercises the exact combination (fire-and-forget env.QUEUE.send() + 302) via a self-service binding, so it reproduces inside a single worker with no miniflare (where this was originally observed).

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[jutaz]

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