test: cover Writable.toWeb() queuing strategy size

[dobbydobap]

Writable.toWeb() sizes queued chunks with a size() function for
non-object-mode streams, added in ca406b2 (#62986, fixing #56269), so that
desiredSize reflects byte length rather than counting each chunk as 1. That
landed without a test, and nothing else covers it:
test-webstreams-adapters-writable-buffer-sources.js only checks chunk
conversion, there is a readable-strategy-option test but no writable
equivalent, and nothing in test/parallel combines Writable.toWeb with
desiredSize.

This covers both branches of the strategy: byte length in non-object mode, one
per chunk in object mode.

Verified in both directions. On v24.18.0, which predates the fix, the
non-object-mode case fails with 99 !== 97 while the object-mode case still
passes, so the test would actually catch a regression instead of passing
regardless. On a main build (v27.0.0-nightly20260831) both pass, including
through tools/test.py.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.06%. Comparing base (242909f) to head (0025851).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65695   +/-   ##
=======================================
  Coverage   90.06%   90.06%           
=======================================
  Files         754      754           
  Lines      256395   256395           
  Branches    48500    48494    -6     
=======================================
+ Hits       230922   230933   +11     
+ Misses      16574    16572    -2     
+ Partials     8899     8890    -9     

see 30 files with indirect coverage changes

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