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

[doma17]

Adds focused csv.writer coverage for QUOTE_STRINGS and QUOTE_NOTNULL.

The existing test_csv coverage already checks basic output for these quote styles. This adds narrower edge-case coverage for:

• rejecting quotechar=None when those quote styles enable quoting
• preserving quote/escape handling with escapechar and doublequote=False

This helps lock down behavior around the newer quoting modes and gives alternative implementations more precise regression coverage.

Testing:

• ./python.exe -m test test_csv -v
• ./python.exe -m test test_csv -q

• Issue: Add csv writer tests for QUOTE_STRINGS and QUOTE_NOTNULL edge cases #151526

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[StanFromIreland]

The quotechar=None check in Modules/_csv.c is:

cpython/Modules/_csv.c

Lines 542 to 546 in 228b1bf

	if (self->quoting != QUOTE_NONE && self->quotechar == NOT_SET) {
	PyErr_SetString(PyExc_TypeError,
	"quotechar must be set if quoting enabled");
	goto err;
	}

so it's mode-agnostic (looping over three quoting modes tests the same branch three times).

The escapechar/doublequote handling in join_append_data is likewise shared by every mode other than QUOTE_NONE:

cpython/Modules/_csv.c

Lines 1219 to 1232 in 228b1bf

	if (dialect->quoting == QUOTE_NONE)
	want_escape = 1;
	else {
	if (c == dialect->quotechar) {
	if (dialect->doublequote)
	ADDCH(dialect->quotechar);
	else
	want_escape = 1;
	}
	else if (c == dialect->escapechar) {
	want_escape = 1;
	}
	if (!want_escape)
	*quoted = 1;

so test_write_quote_styles_escape doesn't exercise anything specific to QUOTE_STRINGS/QUOTE_NOTNULL either. The only mode-specific behaviour is the initial quoted flag set in csv_writerow_lock_held, and that's already covered by the existing ['a','',None,1] cases in test_write_arg_valid.

Seeing as this adds no new coverage, I'm closing.