4.x: Streamable map, filter, flatMap impl, fixes and reworks

[akarnokd]

• Streamer no longer implements AutoCloseable because it implied you could cancel it anytime. But that's what the DisposableContainers are for.

Without proper async support, I have to write the state machine manually, usually using the same wip-trampoline loops.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.693 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.572
src/main/java/io/reactivex/rxjava4/core/Streamer.java: 4.585
src/main/java/io/reactivex/rxjava4/internal/fuseable/HasUpstreamStreamableSource.java: 4.915
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableEmpty.java: 4.881
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromArray.java: 4.651
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterable.java: 4.639
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromStream.java: 4.949
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableHide.java: 4.842
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableJust.java: 4.671
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableNever.java: 4.846
src/main/java/io/reactivex/rxjava4/observers/BaseTestConsumer.java: 4.500
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableForEachTest.java: 4.369
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromIterableTest.java: 4.845
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableFromStreamTest.java: 4.830
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableTest.java: 4.580
src/test/java/io/reactivex/rxjava4/internal/schedulers/SharedSchedulerIsolatedTest.java: 4.838
src/test/java/io/reactivex/rxjava4/internal/schedulers/SharedSchedulerTest.java: 4.667
src/test/java/io/reactivex/rxjava4/schedulers/TestSchedulerTest.java: 4.635
src/test/java/io/reactivex/rxjava4/subscribers/TestSubscriberTest.java: 4.457
src/test/java/io/reactivex/rxjava4/testsupport/BaseTestConsumerEx.java: 4.490

✅ No secrets or suspicious high-entropy strings found.

Mid-4 beauty heuristic in action — powered by our entropy chats! 😊

[codecov]

Codecov Report

❌ Patch coverage is 99.53488% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 98.63%. Comparing base (f68a855) to head (0acd24a).

Files with missing lines	Patch %	Lines
...ternal/operators/streamable/StreamableFlatMap.java	99.03%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8208      +/-   ##
============================================
+ Coverage     98.59%   98.63%   +0.04%     
- Complexity     6884     6898      +14     
============================================
  Files           786      790       +4     
  Lines         48703    48892     +189     
  Branches       6578     6608      +30     
============================================
+ Hits          48019    48225     +206     
+ Misses          496      481      -15     
+ Partials        188      186       -2     

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