stream: fix drop-newest behavior in share()

[trivikr]

Fixes: #64416

Fixes share() with backpressure: 'drop-newest' so upstream pull
results are discarded when the shared buffer reaches highWaterMark.

Previously, #waitForBufferSpace() returned true for drop-newest,
causing the pulled batch to be appended to the full buffer. The buffer
could therefore grow beyond highWaterMark, and stalled consumers
eventually received every batch.

The async implementation now pulls and discards new results while the
buffer is full. The existing test has been updated to verify that the
buffer remains bounded and dropped batches are not delivered.

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.23%. Comparing base (ab41cf0) to head (baae906).

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/share.js	71.42%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #64417   +/-   ##
=======================================
  Coverage   90.23%   90.23%           
=======================================
  Files         741      741           
  Lines      241339   241339           
  Branches    45464    45474   +10     
=======================================
+ Hits       217766   217780   +14     
+ Misses      15132    15122   -10     
+ Partials     8441     8437    -4     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/share.js	84.99% <71.42%> (+0.26%)	⬆️

... and 28 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.