fix: stabilise JDK 25 nightly flakes

[He-Pin]

Motivation:
The JDK 25 nightly workflow run 24922457328 failed only in the JDK 25 matrix. The failures were timeout-shaped: stream TCK subscription/sequential-signal stalls, remote system-message/send-consistency timeouts, BoundedSourceQueue burst delivery, and DurableStateBehaviorStashOverflowSpec draining 20k messages. The existing minimum-runnable=4 mitigation was not enough on the GitHub runners.

References:

• Refs nightly tests (CI): java 25 runs have a lot of stream test failures #2573
• Refs ForkJoinPool compensation-thread regression (JDK-8300995) causes test flakiness on JDK 21+ #2870
• Nightly failure: https://github.com/apache/pekko/actions/runs/24922457328
• Related context: test(stream-tests): enable virtualized stream dispatcher in JDK 21+ nightly CI #2882 and fix: stabilise JDK 21+ / JDK 25 nightly test runs #2889

Modification:

• Export the virtual-thread JVM opens in the JDK 21+ nightly test step through JDK_JAVA_OPTIONS.
• Enable fork-join dispatcher virtualize=on in JDK 21+ nightly builds for actor default, actor internal, remote default, remote classic backoff, persistence default replay, persistence default stream, and stream test dispatchers.
• Keep the JDK 21+ minimum-runnable override, using 8 for JDK 25 nightly runs and 4 for JDK 21.
• Document the nightly virtual-thread override, the required add-opens values, the system-property opt-in for dispatcher virtualize, and the fact that fork-join dispatcher virtualization uses the owning dispatcher fork-join pool as the virtual-thread scheduler.
• Dilate two hard timeout assumptions exposed by the JDK 25 run: BoundedSourceQueueSpec burst delivery and DurableStateBehaviorStashOverflowSpec stash draining.

Result:
The production reference configuration remains unchanged; virtual threads are enabled only by nightly workflow parameters. The selected fork-join dispatchers retain their own scheduler isolation instead of routing through the JVM-wide default virtual-thread scheduler. This targets the CI-only JDK 25 starvation symptoms while preserving local and PR-test defaults.

Tests:

• git diff --check
• sbt -Dpekko.test.timefactor=4 -Dpekko.actor.testkit.typed.timefactor=4 -Dpekko.actor.default-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.actor.internal-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.remote.default-remote-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.test.stream-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.actor.default-dispatcher.fork-join-executor.virtualize=on -Dpekko.actor.internal-dispatcher.fork-join-executor.virtualize=on -Dpekko.remote.default-remote-dispatcher.fork-join-executor.virtualize=on -Dpekko.test.stream-dispatcher.fork-join-executor.virtualize=on "stream-tests / Test / testOnly org.apache.pekko.stream.scaladsl.BoundedSourceQueueSpec" "stream-tests-tck / Test / testOnly org.apache.pekko.stream.tck.FanoutPublisherTest" "persistence-typed-tests / Test / testOnly org.apache.pekko.persistence.typed.state.scaladsl.DurableStateBehaviorStashOverflowSpec"
• sbt -Dpekko.test.timefactor=4 -Dpekko.actor.testkit.typed.timefactor=4 -Dpekko.actor.default-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.actor.internal-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.remote.default-remote-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.test.stream-dispatcher.fork-join-executor.minimum-runnable=8 -Dpekko.actor.default-dispatcher.fork-join-executor.virtualize=on -Dpekko.actor.internal-dispatcher.fork-join-executor.virtualize=on -Dpekko.remote.default-remote-dispatcher.fork-join-executor.virtualize=on -Dpekko.test.stream-dispatcher.fork-join-executor.virtualize=on "remote / Test / testOnly org.apache.pekko.remote.artery.ArteryUpdSendConsistencyWithOneLaneSpec org.apache.pekko.remote.artery.ArteryUpdSendConsistencyWithThreeLanesSpec org.apache.pekko.remote.ArteryMessageLoggingSpec org.apache.pekko.remote.artery.SystemMessageDeliverySpec"
• sbt "persistence-typed-tests / Test / scalafmtCheck" "stream-tests / Test / scalafmtCheck" "docs / Test / scalafmtCheck"
• sbt "docs / Test / scalafmtCheck"

[He-Pin]

@pjfanning I will take another run about this