Fix Streamable.forEach BiConsumer overload ignoring executor

[cuishuang]

This PR fixes the BiConsumer overload of Streamable.forEach() ignoring its ExecutorService.

The overload currently calls CompletableFuture.supplyAsync(...) without passing the provided executor. As a result, the blocking stream consumption runs on ForkJoinPool.commonPool() instead of the executor supplied by the caller.

This also affects Streamable.transform(transformer, executor), which delegates to the same BiConsumer overload.

[github-actions]

🐷 TruffleHog + Entropy Beauty Scan

Average entropy of changed code: 4.367 bits/char
Verdict: ✅ Mid-4 beauty detected (thoughtful human code!)

Changed files entropy:

src/main/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableForEach.java: 4.148
src/test/java/io/reactivex/rxjava4/internal/operators/streamable/StreamableForEachTest.java: 4.587

✅ 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 98.58%. Comparing base (01bd76d) to head (0ded644).

Additional details and impacted files

@@             Coverage Diff              @@
##                4.x    #8302      +/-   ##
============================================
- Coverage     98.62%   98.58%   -0.05%     
  Complexity     7157     7157              
============================================
  Files           824      824              
  Lines         50800    50800              
  Branches       6913     6913              
============================================
- Hits          50104    50080      -24     
- Misses          474      487      +13     
- Partials        222      233      +11     

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