Bug description

Lib/test/test_csv.py already covers the basic writer behavior for csv.QUOTE_STRINGS and csv.QUOTE_NOTNULL, but it does not explicitly exercise a couple of edge cases for these newer quoting modes:

• quotechar=None should be rejected when quoting is enabled for QUOTE_STRINGS and QUOTE_NOTNULL, just like QUOTE_ALL.
• quoted fields using these modes should continue to handle embedded quote characters and escape characters correctly, including the doublequote=False path.

These are small conformance checks for behavior already implemented in _csv, and they help alternative Python implementations catch regressions around the newer quote styles.

Reproducer / expected behavior

import csv
from io import StringIO

for quoting in (csv.QUOTE_STRINGS, csv.QUOTE_NOTNULL):
    try:
        csv.writer(StringIO(), quoting=quoting, quotechar=None)
    except TypeError:
        pass
    else:
        raise AssertionError("quotechar=None should be rejected")

csv.writer should also preserve the existing escaping behavior for QUOTE_STRINGS and QUOTE_NOTNULL when escapechar is configured and doublequote is toggled.

CPython versions tested on

• CPython main branch

Operating systems tested on

• macOS

Linked PRs

• gh-151526: Add csv writer tests for quote style edge cases #151527