stream: drop noop .then in pipe orchestration

[mcollina]

Follow-up to the per-chunk markPromiseAsHandled substitution. Apply the same pattern to the four remaining pipe-orchestration setPromiseHandled call sites that fire once per pipe setup (rather than per chunk):

• readablestream.js: pipeThrough's inner readableStreamPipeTo promise
• readablestream.js: pipeTo's run() loop promise
• transfer.js: cross-realm readable + writable pipe promises

In each case the chain Promise + noop closure that setPromiseHandled allocates has no observer beyond V8's unhandled-rejection tracker — errors flow back via the relevant stream's closed/errored state path, which surrounding code already monitors. markPromiseAsHandled flips the same V8 flag with no allocation.

These sites are not on a per-chunk hot path, so no benchmark impact is expected. The change is purely consistency cleanup matching the per-chunk fix.

WPT streams parity preserved: 1403 subtests passing, 0 unexpected failures.

Other setPromiseHandled call sites in writablestream.js (Ensure{Ready,Closed}PromiseRejected, setupWritableStreamDefaultWriter, RejectCloseAndClosedPromiseIfNeeded) and readablestream.js (readableStreamError, readableStreamReaderGenericRelease) were considered but not converted: probing one of them (Ensure{Ready,Closed}PromiseRejected on the writer-release path) reproduces an unhandled-rejection regression on writable-streams/close.any.js and the piping error-propagation tests — MarkAsHandled and .then(undefined, () => {}) are not equivalent in that specific V8 path. Those sites stay on setPromiseHandled.

[mcollina]

Closing — no measurable perf benefit. These four sites fire once per pipe setup, not per chunk, so the consistency cleanup doesn't justify reviewer burden. The per-chunk equivalent change ships in #63572.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.34%. Comparing base (74ccf38) to head (3bb877e).
⚠️ Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63578      +/-   ##
==========================================
+ Coverage   90.32%   90.34%   +0.01%     
==========================================
  Files         730      730              
  Lines      234209   234483     +274     
  Branches    43934    43908      -26     
==========================================
+ Hits       211558   211837     +279     
+ Misses      14372    14371       -1     
+ Partials     8279     8275       -4     

Files with missing lines	Coverage Δ
lib/internal/webstreams/readablestream.js	98.55% <100.00%> (+<0.01%)	⬆️
lib/internal/webstreams/transfer.js	98.11% <100.00%> (+0.03%)	⬆️
lib/internal/webstreams/util.js	99.57% <100.00%> (+<0.01%)	⬆️

... and 46 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.