4.x: Isolate Maybe takeUntil race tests

[JimmyLiuJia]

Related #8075.

Move onErrorRace and onCompleteRace from MaybeTakeUntilTest and MaybeTakeUntilPublisherTest into corresponding @Isolated classes, following the naming requested in #8075.

The four methods retain their original bodies, loop counts, assertions, and plugin cleanup. Both new classes still extend RxJavaTest. This batch contains only test movements.

Local verification on Temurin 26.0.2.1+1 and Gradle 9.8.0:

• Scoped Maybe takeUntil tests: 35 passed before and after the movement, with zero failures or errors. Only the inherited disabled announce cases were skipped (2 before, 4 after).
• test --tests "io.reactivex.rxjava4.validators.*": 272 passed, 27 skipped, zero failures or errors.

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: 4.769 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeTakeUntilIsolatedTest.java: 4.803
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeTakeUntilPublisherIsolatedTest.java: 4.800
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeTakeUntilPublisherTest.java: 4.795
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeTakeUntilTest.java: 4.680

✅ 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 98.63%. Comparing base (99a9189) to head (ebca2ac).
⚠️ Report is 2 commits behind head on 4.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                4.x    #8319   +/-   ##
=========================================
  Coverage     98.62%   98.63%           
- Complexity     7157     7158    +1     
=========================================
  Files           824      824           
  Lines         50801    50822   +21     
  Branches       6914     6920    +6     
=========================================
+ Hits          50102    50127   +25     
- Misses          474      476    +2     
+ Partials        225      219    -6     

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