4.x: Isolate Maybe zip race tests

[JimmyLiuJia]

Related #8075.

Move the existing innerErrorRace test from both MaybeZipArrayTest and MaybeZipIterableTest into their corresponding @Isolated classes, following the naming requested in #8075.

Both test methods retain their original bodies, loop counts, assertions, and plugin cleanup. The addString helpers remain available to the ordinary tests and are copied into the new classes. This change contains only test movements.

Local verification on Temurin 26.0.2.1+1 and Gradle 9.8.0, using cached dependencies in offline mode:

• Scoped Maybe.zip tests: 28 passed before and 28 passed after the movement, with zero failures or errors. The inherited disabled announce cases were skipped (2 before, 4 after).
• Validators: 272 passed, 27 skipped, zero failures or errors.
• checkstyleTest passed.

This prepares these tests for JUnit isolation when parallel execution is enabled. The current build does not enable JUnit parallel execution, and @Isolated does not coordinate separate Gradle test JVMs.

AI disclosure: OpenAI Codex was used to prepare the changes, run the local checks, and draft this PR description.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.855 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeZipArrayIsolatedTest.java: 4.840
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeZipArrayTest.java: 4.854
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeZipIterableIsolatedTest.java: 4.820
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeZipIterableTest.java: 4.905

✅ No secrets or suspicious high-entropy strings found.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (6539157) to head (fd2005e).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8320      +/-   ##
============================================
+ Coverage     98.61%   98.66%   +0.04%     
+ Complexity     7155     7154       -1     
============================================
  Files           824      824              
  Lines         50822    50822              
  Branches       6920     6920              
============================================
+ Hits          50120    50143      +23     
+ Misses          476      465      -11     
+ Partials        226      214      -12     

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