The ConPTY guard swallowed a chunk only when it exactly equaled BSU or
ESU, so any write concatenating them with other output reached Windows
raw and hung ConPTY, which buffers the unimplemented 2026 sequence
indefinitely (#195; the class recurred in #863's resize path). Strip
every occurrence from string chunks instead: standalone escapes are
swallowed, concatenated ones lose only the escapes, and a swallowed
write now also honors its callback so callback-style writers cannot
wedge. Non-string chunks pass through untouched.
