4.x: Streamable +collect +blocking +withCancel

[akarnokd]

Implement more operators

• blockingFirst(T) and blockingFirst(T, StreamerCancellation) - get the first item or return the default item.
• blockingLast(T) and blockingLast(T, StreamerCancellation) - get the last item or return the default item.
• collect(Supplier, BiConsumer) - collect with explicit supplier and biconsumer callbacks.
• error(Supplier) - generate the error when streaming.
• withCancellation(StreamerCancellation) + 1. Allows injecting a cancellation opportunity into the flow.

Bugfix

• Fix a NPE crash when running a DeferredEnumerableSource with collect and a failing source.

Cleanup

• Organize imports
• Suppress resource warnings in unit tests.

Related

#8299

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.652 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/core/Streamable.java: 4.648
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableBlocking.java: 4.550
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableCollect.java: 3.973
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableCollector.java: 4.042
src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableWithCancellation.java: 4.847
src/test/java/io/reactivex/rxjava4/internal/operators/maybe/MaybeAmbTest.java: 4.633
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableBlockingFirstTest.java: 4.935
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableBlockingLastTest.java: 4.917
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableCollectTest.java: 4.760
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableCollectorTest.java: 4.767
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableErrorTest.java: 4.772
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableWithCancellationTest.java: 4.884
src/test/java/io/reactivex/rxjava4/internal/schedulers/SchedulerToExecutorServiceTest.java: 4.751

✅ 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 97.94521% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.61%. Comparing base (798dc01) to head (74599a3).

Files with missing lines	Patch %	Lines
...ternal/operators/streamable/StreamableCollect.java	97.34%	1 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8333      +/-   ##
============================================
- Coverage     98.63%   98.61%   -0.03%     
- Complexity     7157     7170      +13     
============================================
  Files           824      826       +2     
  Lines         50832    50972     +140     
  Branches       6920     6944      +24     
============================================
+ Hits          50139    50266     +127     
- Misses          473      479       +6     
- Partials        220      227       +7     

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