4.x: Introduce Streamable / Streamer after IAsyncEnumerable / IAsyncEnumreator

[akarnokd]

Based on C#'s IAsyncEnumerables and using Virtual Threads as the means for blocking for results.

In progress. Will add more operators and interop as I go on.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.668 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

.github/workflows/release-notify-x.yml: 4.668

✅ 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 64.28571% with 30 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (4.x@ba12301). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ain/java/io/reactivex/rxjava4/core/Streamable.java	58.06%	12 Missing and 1 partial ⚠️
.../internal/operators/streamable/StreamableJust.java	70.58%	8 Missing and 2 partials ⚠️
...nternal/operators/streamable/StreamableHelper.java	50.00%	3 Missing and 1 partial ⚠️
...a/io/reactivex/rxjava4/disposables/Disposable.java	0.00%	2 Missing ⚠️
...internal/operators/streamable/StreamableEmpty.java	83.33%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (64.28%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@          Coverage Diff           @@
##             4.x    #8048   +/-   ##
======================================
  Coverage       ?   99.52%           
  Complexity     ?     6810           
======================================
  Files          ?      763           
  Lines          ?    47986           
  Branches       ?     6430           
======================================
  Hits           ?    47756           
  Misses         ?      117           
  Partials       ?      113           

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