4.x: Cleanup: Serial annotation, getFirst usage

[akarnokd]

IntelliJ inspections:

• Add @Serial to serialVersionUID fields.
• Replace get(0) with getFirst() where possible.
• Use x instanceof Y z pattern.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.635 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/test/java/io/reactivex/rxjava4/core/SchedulerMainTest.java: 5.033
src/test/java/io/reactivex/rxjava4/disposables/CompositeDisposableTest.java: 4.514
src/test/java/io/reactivex/rxjava4/disposables/SequentialDisposableTest.java: 4.551
src/test/java/io/reactivex/rxjava4/disposables/SerialDisposableTests.java: 4.543
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/BlockingFlowableNextTest.java: 4.520
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRefCountTest.java: 4.572
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableReplayEagerTruncateTest.java: 4.744
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableReplayTest.java: 4.666
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRetryTest.java: 4.531
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableRetryWithPredicateTest.java: 4.663
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableScanTest.java: 4.561
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableSequenceEqualTest.java: 4.651
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableSingleTest.java: 4.614
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableSkipWhileTest.java: 4.937
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableSubscribeOnTest.java: 4.541
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableSwitchIfEmptyTest.java: 4.696
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableSwitchTest.java: 4.678
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableTakeLastTest.java: 4.566
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableTakeTest.java: 4.665
src/test/java/io/reactivex/rxjava4/internal/operators/flowable/FlowableTakeTest2.java: 4.610

✅ 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 98.32%. Comparing base (4d304b9) to head (7c1b500).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8180      +/-   ##
============================================
- Coverage     98.33%   98.32%   -0.02%     
+ Complexity     6940     6939       -1     
============================================
  Files           785      785              
  Lines         49015    49003      -12     
  Branches       6603     6603              
============================================
- Hits          48200    48181      -19     
- Misses          623      624       +1     
- Partials        192      198       +6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.