stream: fix early drain after Utf8Stream reopen

[mcollina]

If a 'ready' listener starts a write after reopen(), the reopen path still emits 'drain' from a nextTick before that write completes, so a listener reading the file on 'drain' can observe it empty. This is the underlying race behind the test-fastutf8stream-reopen flake that 2e8a4b1 deflaked on the test side ("The reopen path also emits a 'drain' of its own from a nextTick before the write has landed") — this fixes the library so 'drain' is never emitted while stream.writing === true.

Skip the extra emit when a write is in flight: #release() emits the real 'drain' once the write completes, so no event is lost.

The regression test defers the reopened file's fs.write by one setImmediate, which deterministically lands the write after the nextTick on which the premature 'drain' used to fire. It fails without the lib change (asserts writing === false on 'drain'; observed true, file empty) and passes with it.

Verified with 800 runs of the test under 16× parallel CPU contention: 0 failures (28/800 before the fix, reproducing the CI signature exactly).

Same fix submitted to upstream sonic-boom: pinojs/sonic-boom#255.

[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 (7b0de5e) to head (7837e2f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #65633   +/-   ##
=======================================
  Coverage   90.05%   90.05%           
=======================================
  Files         754      754           
  Lines      255722   255724    +2     
  Branches    48314    48310    -4     
=======================================
+ Hits       230281   230286    +5     
- Misses      16555    16560    +5     
+ Partials     8886     8878    -8     

Files with missing lines	Coverage Δ
lib/internal/streams/fast-utf8-stream.js	80.86% <100.00%> (+0.04%)	⬆️

... and 34 files with indirect coverage changes

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