Curling a pod with failed readinessProbe should not hang

## What version of Knative?

all recent versions

## Expected Behavior

Curling a pod with a readiness probe set (with periodSeconds=0 to avoid the behaviour in https://github.com/knative/serving/issues/10764), should either error immediately or succeed due to us scaling things up / terminating the pod (needs discussion about what the right behaviour is!).

## Actual Behavior

Curling a pod with a failed readiness check hangs (unless there are other scaled-up pods for the request). The pod is never killed, because there is no liveness probe, but the set of routable pods is empty. In upstream k8s you would get a "no healthy upstream" error, in knative your curl hangs 😢.

## Steps to Reproduce the Problem

1. `ko apply -f https://github.com/julz/readybutnotsteady/blob/main/service-no-period.yaml`
1. Wait for startup to succeed
1. `curl readynotsteady.default.$domain/start-failing`
1. Wait a few seconds for the pod to go unready
1. `curl readynotsteady.default.$domain` (hangs).

Apply service-no-period.yaml from the readybutnotsteady reproduction, wait for startup, trigger /start-failing, and then curl the service after the pod becomes unready. Trace how the request is handled when no routable pods remain; done means the request either fails immediately or succeeds according to the behavior agreed by the maintainers.

Written by the indexing model from the issue text.