4.x: Upgrade to Java 26 baseline compilation, test isolation

[akarnokd]

I installed Oracle JDK 26 locally for now.

Also hopefully fixes a failing test via @Isolated running: CompletableTest.repeatNormal.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.993 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

README.md: 4.993

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[akarnokd]

WARNING: Final field mockDisposable in class io.reactivex.rxjava4.internal.operators.flowable.FlowableReplayTest$InprocessWorker has been mutated reflectively by class org.mockito.internal.util.reflection.ReflectionMemberAccessor in unnamed module @7b36aa0c (file:/C:/Users/akarnokd/.gradle/caches/modules-2/files-2.1/org.mockito/mockito-core/4.11.0/ce5226440c2ee78915716d4ce3d10aed2dbf26fb/mockito-core-4.11.0.jar)
WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
WARNING: Mutating final fields will be blocked in a future release unless final field mutation is enabled
WARNING: Final field mockDisposable in class io.reactivex.rxjava4.internal.operators.flowable.FlowableReplayEagerTruncateTest$InprocessWorker has been mutated reflectively by class org.mockito.internal.util.reflection.ReflectionMemberAccessor in unnamed module @7b36aa0c (file:/C:/Users/akarnokd/.gradle/caches/modules-2/files-2.1/org.mockito/mockito-core/4.11.0/ce5226440c2ee78915716d4ce3d10aed2dbf26fb/mockito-core-4.11.0.jar)
WARNING: Use --enable-final-field-mutation=ALL-UNNAMED to avoid a warning
WARNING: Mutating final fields will be blocked in a future release unless final field mutation is enabled

Will fix this in a separate PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (4.x@7b0fddf). Learn more about missing BASE report.

Additional details and impacted files

@@          Coverage Diff           @@
##             4.x    #8074   +/-   ##
======================================
  Coverage       ?   99.29%           
  Complexity     ?     6829           
======================================
  Files          ?      764           
  Lines          ?    48246           
  Branches       ?     6470           
======================================
  Hits           ?    47906           
  Misses         ?      209           
  Partials       ?      131           

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