stream: validate broadcast writer writev chunks

[trivikr]

Fixes BroadcastWriter writev() and writevSync() input validation so
non-array chunks values throw ERR_INVALID_ARG_TYPE before reaching
convertChunks().

Previously, writer.writev('abc') treated the string as array-like and wrote
abc, while writer.writev(42) resolved as an empty batch.

Fixes: #63299

---

Assisted-by: openai:gpt-5.5

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.04%. Comparing base (c24e552) to head (11dc64d).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63300      +/-   ##
==========================================
- Coverage   90.05%   90.04%   -0.01%     
==========================================
  Files         714      714              
  Lines      225247   225253       +6     
  Branches    42578    42581       +3     
==========================================
- Hits       202842   202830      -12     
- Misses      14181    14220      +39     
+ Partials     8224     8203      -21     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/broadcast.js	84.76% <100.00%> (+0.11%)	⬆️

... and 34 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73438/