stream: fix UTF-8 character corruption in fast-utf8-stream

[mcollina]

Summary

Fix releaseWritingBuf() to correctly handle partial writes that split multi-byte UTF-8 characters.

The previous implementation incorrectly converted byte counts to character counts by using:

n = Buffer.from(writingBuf).subarray(0, n).toString().length;

When n bytes cuts through a multi-byte character, the incomplete UTF-8 sequence becomes U+FFFD (replacement character) via .toString(), which has a different .length than the original character. This caused:

• 3-byte characters (CJK) to be silently dropped
• 4-byte characters (emoji) to leave lone surrogates in the buffer

The fix backs up from the byte position to find a valid UTF-8 character boundary by checking for continuation bytes (pattern 10xxxxxx), then decodes the properly-aligned bytes to get the correct character count.

Also fixes a typo where this._asyncDrainScheduled was used instead of the private field this.#asyncDrainScheduled.

Fixes: #61744

[nodejs-github-bot]

Review requested:

• @nodejs/streams

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.74%. Comparing base (f77a709) to head (4f5cf65).
⚠️ Report is 44 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #61745      +/-   ##
==========================================
- Coverage   89.74%   89.74%   -0.01%     
==========================================
  Files         674      675       +1     
  Lines      204360   204537     +177     
  Branches    39265    39308      +43     
==========================================
+ Hits       183412   183569     +157     
- Misses      13251    13257       +6     
- Partials     7697     7711      +14     

Files with missing lines	Coverage Δ
lib/internal/streams/fast-utf8-stream.js	74.57% <100.00%> (+0.33%)	⬆️

... and 44 files with indirect coverage changes

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