stream: validate fromWritable() options before cache

[trivikr]

Fixes stream/iter fromWritable() cache handling so later options are
validated and applied correctly.

Previously, fromWritable(writable) cached a strict writer, and later calls
such as fromWritable(writable, { backpressure: 'drop-newest' }) returned that
cached strict writer before validating or applying the new options. This also
meant invalid later options did not throw.

This updates fromWritable() to validate options before cache lookup and cache
writers by both Writable instance and backpressure policy.

Fixes: #63277

---

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 90.03%. Comparing base (78bbee3) to head (eb326fb).
⚠️ Report is 18 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63278      +/-   ##
==========================================
- Coverage   90.04%   90.03%   -0.02%     
==========================================
  Files         713      714       +1     
  Lines      225003   225250     +247     
  Branches    42536    42576      +40     
==========================================
+ Hits       202606   202805     +199     
- Misses      14177    14218      +41     
- Partials     8220     8227       +7     

Files with missing lines	Coverage Δ
lib/internal/streams/iter/classic.js	89.44% <100.00%> (+0.09%)	⬆️

... and 43 files with indirect coverage changes

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