[PlaintextExtractor] Verify handling files missing newline delimiters

A file may end without a newline-like delimiter, either as it has none,
or if the file uses something else (e.g. classic Mac OS used '\r' only).

The extractor behavior here depends on which implementation is used:
- std::getline will strip the default delimiter (i.e. '\n') and return the
  remainder
- getline will keep the delimiter, but the code unconditionally removes
  the last byte from the line. This may truncate actual content.