Fix LineBuffer payload byte accounting

[Sagar07-star]

Summary
Count the UTF-8 payload bytes, including newline separators, when deciding whether LineBuffer should flush.
Problem
LineBuffer currently counts only characters in each input line, while SocketWriter sends the newline-joined buffer encoded as UTF-8. For example, two three-character lines produce seven payload bytes but count as six, so a seven-byte threshold does not trigger a flush.
Changes

• Include one separator byte for each line after the first.
• Count encoded UTF-8 bytes instead of characters.
• Add regression coverage for separator-triggered flushing, multibyte text, empty lines, and size reset after flushing.
  The threshold still triggers after appending a whole line; this does not introduce a strict datagram size cap.
  Validation
• All four LineBuffer tests fail before the fix and pass afterward (including the corrected existing size expectation).
• Python 3.12: python -m unittest discover -s tests -t . -v: 121 passed, two Unix-domain-socket tests blocked by PermissionError in the execution environment.
• git diff --check passes.