4.x: Streamable Ops: doOnNext, intercept, lift, to; test cleanup

[akarnokd]

Preparing for better lifecycle management and verification.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.607 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.566
src/main/java/io/reactivex/rxjava4/core/Streamer.java: 4.621
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFilter.java: 4.303
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFlatMap.java: 4.183
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMap.java: 4.566
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMapOptional.java: 4.364
src/main/java/io/reactivex/rxjava4/observers/BaseTestConsumer.java: 4.499
src/test/java/io/reactivex/rxjava4/internal/operators/observable/ObservableConcatMapSchedulerTest.java: 4.857
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableErrorTest.java: 4.833
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFilterTest.java: 4.737
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFlatMapTest.java: 4.785
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMapOptionalTest.java: 4.814
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableMapTest.java: 4.910
src/test/java/io/reactivex/rxjava4/subscribers/TestSubscriberTest.java: 4.456

✅ 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 98.61%. Comparing base (bb26a05) to head (3fe835f).
⚠️ Report is 1 commits behind head on 4.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8209      +/-   ##
============================================
- Coverage     98.62%   98.61%   -0.02%     
- Complexity     6899     6910      +11     
============================================
  Files           790      793       +3     
  Lines         48892    48948      +56     
  Branches       6608     6609       +1     
============================================
+ Hits          48219    48268      +49     
- Misses          484      486       +2     
- Partials        189      194       +5     

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