stream: validate fromWritable() options before cache

[trivikr]

Fixes stream/iterfromWritable() 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 90.05%. Comparing base (2edd842) to head (6a2e3d3).
⚠️ Report is 13 commits behind head on main.

Additional details and impacted files

@@ Coverage Diff @@## main #63278 +/- ##
==========================================
+ Coverage 90.04% 90.05% +0.01% 
==========================================
Files 714 714 Lines 225338 225346 +8 Branches 42598 42613 +15 ==========================================
+ Hits 202897 202933 +36 + Misses 14236 14192 -44 - Partials 8205 8221 +16 

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

... and 32 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73468/

[nodejs-github-bot]

Landed in 2d6cbea