[java-pubsub] PublisherImplTest.testResumePublish hangs

In the release PR run https://github.com/googleapis/google-cloud-java/actions/runs/27152757144/job/80148008212?pr=13373, the unit test `PublisherImplTest` in `google-cloud-pubsub` module got stuck and caused the GHA run to timeout after 6 hours.

Analyzing the job logs of previous timed-out release runs (e.g. https://github.com/googleapis/google-cloud-java/actions/runs/27024407145), the hang consistently occurs inside `com.google.cloud.pubsub.v1.PublisherImplTest` when running on Java 8 runtime.

Specifically, the `testResumePublish` test case invokes `sendTestMessageWithOrderingKey` which triggers a publish call to the mock server `FakePublisherServiceImpl`. Inside `FakePublisherServiceImpl.publish()`, it calls `publishResponses.take()`, which is a blocking queue call. In Java 8 runtime, this blocks the main thread, resulting in a deadlock before `testPublisherServiceImpl.addPublishResponse(...)` can be called.

This test was previously disabled due to similar timeout issues (Issue #13051) but was recently re-enabled. We need to investigate why it deadlocks under Java 8 and prevent it from hanging.