4.x: Isolate Maybe switchIfEmpty race tests

[JimmyLiuJia]

Related #8075.

Move the existing emptyCancelRace tests from MaybeSwitchIfEmptyTest and MaybeSwitchIfEmptySingleTest into MaybeSwitchIfEmptyIsolatedTest and MaybeSwitchIfEmptySingleIsolatedTest.

This is a movements-only batch:

• The test bodies, race loop, assertions, and cleanup are unchanged.
• The original classes retain all of their other tests.
• The new classes follow the requested <Original>IsolatedTest.java naming and use JUnit @Isolated.
• No production code, build configuration, or test-runner configuration changed.

Tests run on Windows with OpenJDK 26.0.2.1 and Gradle 9.8.0:

• gradle test --tests "io.reactivex.rxjava4.internal.operators.maybe.MaybeSwitchIfEmpty*" --stacktrace --no-daemon on the clean base and after the move
• gradle test --tests "io.reactivex.rxjava4.validators.*" --stacktrace --no-daemon
• gradle checkstyleMain --no-daemon --offline
• gradle checkstyleTest --no-daemon
• gradle --gradle-user-home D:\GitHub-Maintenance\tools\gradle-user-home build javadoc --offline --stacktrace --no-daemon
• git diff --check

The full build completed successfully with 5,796 JUnit tests, 4,001 TestNG/TCK tests, Checkstyle, JaCoCo, and Javadoc. The local build used the installed Gradle 9.8.0 binary because the repository wrapper distribution was not cached.

AI assistance was used to prepare and check this change; the submitted author is responsible for the contribution.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.976 bits/char
Verdict: ⚠️ Consider review — entropy outside sweet spot

Changed files entropy:

src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeSwitchIfEmptyIsolatedTest.java: 4.999
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeSwitchIfEmptySingleIsolatedTest.java: 4.994
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeSwitchIfEmptySingleTest.java: 4.959
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeSwitchIfEmptyTest.java: 4.952

✅ 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 98.64%. Comparing base (ddc735d) to head (ba32990).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8328      +/-   ##
============================================
- Coverage     98.64%   98.64%   -0.01%     
  Complexity     7157     7157              
============================================
  Files           824      824              
  Lines         50822    50822              
  Branches       6920     6920              
============================================
- Hits          50134    50133       -1     
- Misses          468      470       +2     
+ Partials        220      219       -1     

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