flaky PublisherSwitchMapDelayErrorTckTest.required_spec313_cancelMustMakeThePublisherEventuallyDropAllReferencesToTheSubscriber

https://github.com/apple/servicetalk/actions/runs/14410532770/job/40417131618#step:8:513
```
Gradle suite > Gradle test > io.servicetalk.concurrent.reactivestreams.tck.PublisherSwitchMapDelayErrorTckTest > required_spec313_cancelMustMakeThePublisherEventuallyDropAllReferencesToTheSubscriber FAILED
java.lang.AssertionError: Publisher io.servicetalk.concurrent.reactivestreams.ReactiveStreamsAdapters$StToRsPublisher@2b40ff9c did not drop reference to test subscriber after subscription cancellation
at org.testng.Assert.fail(Assert.java:110)
at org.reactivestreams.tck.TestEnvironment.flop(TestEnvironment.java:222)
at org.reactivestreams.tck.PublisherVerification$27.run(PublisherVerification.java:1018)
at org.reactivestreams.tck.PublisherVerification.activePublisherTest(PublisherVerification.java:1135)
at org.reactivestreams.tck.PublisherVerification.required_spec313_cancelMustMakeThePublisherEventuallyDropAllReferencesToTheSubscriber(PublisherVerification.java:1007)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)

```

Start by running the Gradle test PublisherSwitchMapDelayErrorTckTest.required_spec313_cancelMustMakeThePublisherEventuallyDropAllReferencesToTheSubscriber and review the linked CI failure. Trace the ReactiveStreamsAdapters$StToRsPublisher subscription-cancellation path; done means the test no longer flakes and the publisher eventually drops its reference to the subscriber.

Written by the indexing model from the issue text.