MapAsyncPartitionedSpec: fix broken test on Java 25

[pjfanning]

https://github.com/apache/pekko/actions/runs/25410349422/job/74530492133

[05-06 01:20:15.659] [info] - should process elements in parallel preserving order in partition *** FAILED *** (4 minutes, 0 seconds)
[05-06 01:20:15.662] [info]   was thrown during property evaluation.
[05-06 01:20:15.662] [info]     Message: A timeout occurred waiting for a future to complete. Waited 240000000000 nanoseconds.
[05-06 01:20:15.662] [info]     Location: (MapAsyncPartitionedSpec.scala:318)
[05-06 01:20:15.662] [info]     Occurred when passed generated values (
[05-06 01:20:15.662] [info]       arg0 = Parallelism(2),
[05-06 01:20:15.662] [info]       arg1 = Vector(TestKeyValue(0, 12 milliseconds, "1"), TestKeyValue(2, 13 milliseconds, "2"), TestKeyValue(3, 15 milliseconds, "3"), TestKeyValue(2, 28 milliseconds, "4"), TestKeyValue(0, 26 milliseconds, "5"), TestKeyValue(3, 19 milliseconds, "6"), TestKeyValue(2, 21 milliseconds, "7"), TestKeyValue(1, 15 milliseconds, "8"), TestKeyValue(1, 23 milliseconds, "9"), TestKeyValue(1, 10 milliseconds, "10"), TestKeyValue(4, 28 milliseconds, "11"), TestKeyValue(1, 11 milliseconds, "12"), TestKeyValue(2, 28 milliseconds, "13"), TestKeyValue(3, 21 milliseconds, "14"), TestKeyValue(3, 10 milliseconds, "15"), TestKeyValue(1, 26 milliseconds, "16"), TestKeyValue(2, 24 milliseconds, "17"), TestKeyValue(4, 28 milliseconds, "18"), TestKeyValue(4, 14 milliseconds, "19"), TestKeyValue(0, 16 milliseconds, "20"), TestKeyValue(2, 22 milliseconds, "21"), TestKeyValue(1, 24 milliseconds, "22"), TestKeyValue(4, 14 milliseconds, "23"), TestKeyValue(2, 10 milliseconds, "24"), TestKeyValue(3, 17 milliseconds, "25"), TestKeyValue(0, 16 milliseconds, "26"), TestKeyValue(4, 29 milliseconds, "27"), TestKeyValue(1, 19 milliseconds, "28"), TestKeyValue(1, 23 milliseconds, "29"), TestKeyValue(2, 20 milliseconds, "30"), TestKeyValue(2, 26 milliseconds, "31"), TestKeyValue(2, 22 milliseconds, "32"), TestKeyValue(1, 25 milliseconds, "33"), TestKeyValue(4, 13 milliseconds, "34"), TestKeyValue(0, 22 milliseconds, "35"), TestKeyValue(3, 25 milliseconds, "36"), TestKeyValue(1, 24 milliseconds, "37"), TestKeyValue(1, 22 milliseconds, "38"), TestKeyValue(4, 18 milliseconds, "39"), TestKeyValue(1, 20 milliseconds, "40"), TestKeyValue(1, 27 milliseconds, "41"), TestKeyValue(3, 24 milliseconds, "42"), TestKeyValue(0, 26 milliseconds, "43"), TestKeyValue(3, 18 milliseconds, "44"), TestKeyValue(1, 22 milliseconds, "45"), TestKeyValue(2, 23 milliseconds, "46"), TestKeyValue(4, 14 milliseconds, "47"), TestKeyValue(2, 26 milliseconds, "48"))
[05-06 01:20:15.662] [info]     )
[05-06 01:20:15.663] [info]   Init Seed: 5676852480423376402
[05-06 01:20:15.663] [info]   org.scalatest.exceptions.GeneratorDrivenPropertyCheckFailedException:
[05-06 01:20:15.663] [info]   at org.scalatestplus.scalacheck.CheckerAsserting$$anon$2.indicateFailure(CheckerAsserting.scala:255)
[05-06 01:20:15.663] [info]   at org.scalatestplus.scalacheck.CheckerAsserting$$anon$2.indicateFailure(CheckerAsserting.scala:242)
[05-06 01:20:15.663] [info]   at org.scalatestplus.scalacheck.UnitCheckerAsserting$CheckerAssertingImpl.check(CheckerAsserting.scala:164)
[05-06 01:20:15.663] [info]   at org.scalatestplus.scalacheck.ScalaCheckDrivenPropertyChecks$ConfiguredPropertyCheck.apply(ScalaCheckDrivenPropertyChecks.scala:631)
[05-06 01:20:15.663] [info]   at org.apache.pekko.stream.MapAsyncPartitionedSpec.$anonfun$new$57(MapAsyncPartitionedSpec.scala:313)
[05-06 01:20:15.663] [info]   at org.scalatest.OutcomeOf.outcomeOf(OutcomeOf.scala:85)

Copilot recommends reducing the pressure for Java 25. It also recommended setting the minimum-runnable config setting for the fork-join-pool to a value like 8 but I've left that out for now. I'm more interested in getting the tests to pass and then we can see about improving them.