Move TestHelper.race tests into @Isolated classes: batch 2 (#8075)

[oaksprout]

Continues the pattern from #8253 onto the Observable flatMap/mergeWith family, the direct counterpart to the Flowable batch already merged. Same shape: TestHelper.race( calls moved into new *IsolatedTest.java classes, @isolated annotation, originals otherwise untouched.

9 methods moved across 6 classes (smaller than batch 1's 15, since Observable has no backpressure protocol, so the request based race variants don't apply here).

Tests: all 12 touched classes, run twice, 0 failures / 0 errors both rounds. Full tree compileTestJava and checkstyleTest both clean. JDK 26.

179 to 173 files with the pattern remain.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.841 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

.github/workflows/gradle_branch.yml: 5.078
.github/workflows/gradle_pr.yml: 4.933
.github/workflows/gradle_preview_jdks.yml: 5.102
.github/workflows/gradle_release.yml: 5.206
.github/workflows/gradle_snapshot.yml: 5.057
.github/workflows/scorecard.yml: 4.765
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapIsolatedTest.java: 4.677
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapMaybeIsolatedTest.java: 4.802
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapMaybeTest.java: 4.714
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapSingleIsolatedTest.java: 4.897
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapSingleTest.java: 4.728
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableFlatMapTest.java: 4.624
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableMergeWithCompletableIsolatedTest.java: 4.873
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableMergeWithCompletableTest.java: 4.712
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableMergeWithMaybeIsolatedTest.java: 4.892
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableMergeWithMaybeTest.java: 4.609
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableMergeWithSingleIsolatedTest.java: 4.882
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableMergeWithSingleTest.java: 4.592

✅ 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 98.56%. Comparing base (ddcbd7b) to head (ab1e8f6).
⚠️ Report is 3 commits behind head on 4.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8257      +/-   ##
============================================
- Coverage     98.63%   98.56%   -0.08%     
+ Complexity     7146     7141       -5     
============================================
  Files           824      824              
  Lines         50768    50768              
  Branches       6905     6905              
============================================
- Hits          50075    50037      -38     
- Misses          476      496      +20     
- Partials        217      235      +18     

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