Port from openclaw/openclaw#112325: multibyte UTF-8 characters split
across a 4096-byte pipe or PTY read boundary were decoded statelessly
per chunk with errors='replace', corrupting both halves into U+FFFD
mojibake in background process output (poll/log/wait/completion
notifications). The foreground path already used an incremental decoder
(tools/environments/base.py::_wait_for_process); this applies the same
treatment to the background reader loops:
- _reader_loop (select and blocking paths): one
codecs.getincrementaldecoder('utf-8') per reader holds partial
sequences across chunks; the finally block flushes a truncated tail
as a single U+FFFD instead of dropping it.
- _pty_reader_loop: same treatment for ptyprocess byte chunks
(pywinpty str chunks pass through unchanged).
Genuinely invalid bytes keep errors='replace' behavior.
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