Flaky Test: PublisherBufferConcurrencyTest

https://github.com/apple/servicetalk/runs/3081403401

```
java.util.concurrent.TimeoutException: largeRun() timed out after 30 seconds
at org.junit.jupiter.engine.extension.TimeoutInvocation.createTimeoutException(TimeoutInvocation.java:70)
...
Suppressed: java.lang.InterruptedException
at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1040)
at java.base/java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1345)
at java.base/java.util.concurrent.CountDownLatch.await(CountDownLatch.java:232)
at io.servicetalk.concurrent.api.SourceToFuture.get(SourceToFuture.java:91)
at io.servicetalk.concurrent.api.PublisherBufferConcurrencyTest.runTest(PublisherBufferConcurrencyTest.java:168)
at io.servicetalk.concurrent.api.PublisherBufferConcurrencyTest.largeRun(PublisherBufferConcurrencyTest.java:53)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.base/java.lang.reflect.Method.invoke(Method.java:566)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutInvocation.proceed(TimeoutInvocation.java:46)
... 83 more
```

Start with PublisherBufferConcurrencyTest, especially largeRun() and runTest() at the stack-trace locations, and reproduce the timeout from the linked CI run. Trace the CountDownLatch wait through SourceToFuture.get() to identify why the test stalls; done means the concurrency test completes reliably without the 30-second timeout.

Written by the indexing model from the issue text.