fix: make AbstractPersistentActorWithTimers and AbstractFSMWithStash subclassable from Java on Scala 3

[pjfanning]

Motivation

Extending AbstractPersistentActorWithTimers from Java fails to compile against the Scala 3 artifacts:

TestActor.java:[5,8] types org.apache.pekko.actor.Timers and org.apache.pekko.persistence.Eventsourced are incompatible;
  class TestActor inherits unrelated defaults for aroundPreRestart(java.lang.Throwable,scala.Option<java.lang.Object>)
  from types org.apache.pekko.actor.Timers and org.apache.pekko.persistence.Eventsourced

Both Timers and Eventsourced implement aroundReceive, aroundPreRestart and aroundPostStop. Scala 3 does emit the mixin forwarder into the class, but flags it ACC_BRIDGE, ACC_SYNTHETIC:

public void aroundPreRestart(java.lang.Throwable, scala.Option);
  flags: (0x1041) ACC_PUBLIC, ACC_BRIDGE, ACC_SYNTHETIC

javac ignores synthetic and bridge members when resolving inherited members, so it falls back to the two interface defaults and rejects the subclass. Scala 2.13 emits the same forwarder without those flags, which is why the 2.12/2.13 artifacts work. This is accidental, not a deliberate limitation.

The reporter asked whether other multi-trait base classes are affected. I compiled a javac probe (an abstract Java subclass) against the Scala 3 build output for every Java-facing Abstract*/UntypedAbstract* class that mixes in more than one trait — the stash, logging and timers variants, AbstractLoggingFSM, AbstractFSMWithStash, AbstractPersistentActor, AbstractPersistentActorWithTimers, AbstractPersistentActorWithAtLeastOnceDelivery, AbstractPersistentFSM, AbstractPersistentLoggingFSM, journal.japi.AsyncWriteJournal, AbstractInOutHandler and AbstractSerializationSupport. Exactly one other class has the same defect: AbstractFSMWithStash, where FSM and UnrestrictedStash both implement postStop.

Modification

Give both classes real (non-synthetic) overrides of the conflicting members that simply delegate to super, which resolves exactly like the forwarders they replace, so behaviour is unchanged on both Scala versions.

AbstractFSMWithStash.postStop deliberately carries no @throws(classOf[Exception]): FSM.postStop declares no checked exceptions, and a wider throws clause is not a valid override for javac.

Added Java sources exercising both classes, so the Scala 3 build fails if this regresses:

• persistence/src/test/java/.../JavaTimerPersistentActor.java, driven by a new case in TimerPersistentActorSpec
• actor-tests/src/test/java/.../AbstractFSMWithStashActorTest.java, a JUnit 5 test

Result

Java subclasses of AbstractPersistentActorWithTimers and AbstractFSMWithStash compile against the Scala 3 artifacts. No other Java-facing multi-trait base class in the build is affected.

Tests

• sbt "++3.3.8" "persistence/testOnly org.apache.pekko.persistence.TimerPersistentActorSpec" — 5 succeeded, 0 failed
• sbt "++3.3.8" "actor-tests/testOnly org.apache.pekko.actor.AbstractFSMWithStashActorTest org.apache.pekko.actor.AbstractFSMActorTest" — 2 succeeded, 0 failed
• sbt "persistence/testOnly org.apache.pekko.persistence.TimerPersistentActorSpec" (2.13) — 5 succeeded, 0 failed
• sbt "actor-tests/testOnly org.apache.pekko.actor.AbstractFSMWithStashActorTest org.apache.pekko.actor.AbstractFSMActorTest" (2.13) — 2 succeeded, 0 failed
• sbt headerCreateAll javafmtAll scalafmtAll scalafmtSbt on JDK 17 — headers generated by sbt, no formatting churn outside the changed files
• sbt +mimaReportBinaryIssues — Not run locally, left to the CI Check / Binary Compatibility job

Both Java test sources fail to compile under Scala 3 without the corresponding production change, and compile once it is applied.

References

Fixes #3474