csv.writer: multi-character lineterminator is truncated to a single byte #8322

Feature

Follow-up from #8302 / #8315.

csv.writer should accept an arbitrary string as lineterminator. RustPython stores it as a single byte, so a multi-character terminator is truncated instead of being emitted in full.

import csv, io
sio = io.StringIO()
csv.writer(sio, lineterminator="!@#").writerow(["a", "b"])
print(repr(sio.getvalue()))
# CPython:    'a,b!@#'
# RustPython: 'a,b!'

Because of this, test_write_lineterminator (Lib/test/test_csv.py) is marked @unittest.expectedFailure — it fails only on the '!@#' subcase.

Scope is limited to the multi-character lineterminator. Left as separate follow-ups: lineterminator validation parity, non-ASCII delimiter/quotechar/escapechar (see #8310), and reader semantics (CPython's reader ignores lineterminator and only recognizes CR/LF).

Python Documentation or reference to CPython source code

https://docs.python.org/3/library/csv.html#csv.Dialect.lineterminator

_{Drafted with AI assistance (Claude Code), reviewed by me.}