stream: drain pending writes before broadcast end

[trivikr]

Fixes: #65333

Keep processing writes that were queued before end(). Signal the broadcast end only after those writes enter the shared buffer.

Resolve end() after all consumers reach end-of-stream. This prevents backpressured writes from remaining pending forever.

---

Assisted-by: codex:gpt-5.6-sol

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 88.04348% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.14%. Comparing base (453f47a) to head (c28857e).
⚠️ Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/streams/iter/broadcast.js	88.04%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65334   +/-   ##
=======================================
  Coverage   90.14%   90.14%           
=======================================
  Files         751      751           
  Lines      252709   252767   +58     
  Branches    47557    47588   +31     
=======================================
+ Hits       227800   227861   +61     
+ Misses      16206    16202    -4     
- Partials     8703     8704    +1     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	88.44% <88.04%> (+0.41%)	⬆️

... and 26 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.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75956/

[trivikr]

Landed in c28857e