stream: fix Utf8Stream flush handling

[jasnell]

Fix several issues with Utf8Stream flushing:

• flush() now writes buffered data regardless of minLength and invokes the callback only after pending writes complete, including when minLength is zero and a write is in flight.
• Multiple concurrent flush() calls are tracked correctly and end() waits for pending flushes before closing.
• flushSync() throws ERR_INVALID_STATE if called while an asynchronous write is in progress instead of corrupting output.
• Periodic flushes no longer stack up while a flush is pending.
• fsync is skipped for stdout/stderr file descriptors.
• reopen(), end() and destroy() behave correctly when the stream is destroyed while still opening.

Separated out from #65840

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

❌ Patch coverage is 87.69231% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.42%. Comparing base (7fab656) to head (8034879).

Files with missing lines	Patch %	Lines
lib/internal/streams/fast-utf8-stream.js	87.69%	16 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #66473   +/-   ##
=======================================
  Coverage   90.42%   90.42%           
=======================================
  Files         791      791           
  Lines      275569   275662   +93     
  Branches    52842    52888   +46     
=======================================
+ Hits       249185   249273   +88     
+ Misses      16802    16800    -2     
- Partials     9582     9589    +7     

Files with missing lines	Coverage Δ
lib/internal/streams/fast-utf8-stream.js	84.14% <87.69%> (+2.77%)	⬆️

... and 24 files with indirect coverage changes

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