ProtocolReader discarded the entire chunk containing the newline
delimiter while draining an oversized message, including bytes after
the newline that belong to the next pipelined message. This corrupted
framing for the rest of the connection (affects server and client).
Salvaged bytes are now kept in a _leftover buffer that read_message()
consumes before touching the stream.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>