stream: fix pipeTo to defer writes per WHATWG spec

[mcollina]

Summary

• Fix PipeToReadableStreamReadRequest[kChunk] to defer writes via queueMicrotask() per the WHATWG Streams spec
• The spec requires that pipeTo's chunk steps must not synchronously call the write algorithm during enqueue()
• Remove expected WPT test failure for "enqueue() must not synchronously call write algorithm"

Details

Previously, PipeToReadableStreamReadRequest[kChunk] would synchronously call writableStreamDefaultWriterWrite(), which violated the WHATWG Streams spec (ReadableStreamPipeTo step 15's "chunk steps").

This caused the WPT test piping/general-addition.any.js → "enqueue() must not synchronously call write algorithm" to fail, which was tracked as an expected failure (see whatwg/streams#1243).

Wrapping the write in queueMicrotask() defers it to the next microtask, matching the spec's required timing semantics.

Refs: whatwg/streams#1243

Test plan

• WPT streams tests pass (test/wpt/test-streams.js)
• Previously expected failure now passes
• All test/parallel/test-webstreams-*.js tests pass

[nodejs-github-bot]

Review requested:

• @nodejs/web-standards

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (ae2ffce) to head (f599463).
⚠️ Report is 69 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61800      +/-   ##
==========================================
- Coverage   89.75%   89.74%   -0.01%     
==========================================
  Files         674      675       +1     
  Lines      204416   204679     +263     
  Branches    39285    39337      +52     
==========================================
+ Hits       183472   183697     +225     
- Misses      13227    13282      +55     
+ Partials     7717     7700      -17     

Files with missing lines	Coverage Δ
lib/internal/webstreams/readablestream.js	98.45% <100.00%> (+<0.01%)	⬆️

... and 66 files with indirect coverage changes

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[jasnell]

I've always thought this one was a bit silly but ok.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71333/

[MattiasBuelens]

Previously, PipeToReadableStreamReadRequest[kChunk] would synchronously call writableStreamDefaultWriterWrite(), which violated the WHATWG Streams spec (ReadableStreamPipeTo step 15's "chunk steps").

Is it just me, or does this not even appear in the spec text? Step 15 doesn't even mention "chunk steps". 🤔

I agree that the specification for ReadableStreamPipeTo leaves a lot of choices up to the implementor, even though the WPT tests require a lot of specific timings...

[MattiasBuelens]

Even the reference implementation skips this test. 😅

Is this extra microtask now a "de facto" standard? Looking at wpt.fyi, all major browsers are already passing this test, even though the spec text doesn't require it...