Temporal: Add ISO8601 PlainDate.p.with leap year test

This was not particularly well covered in the existing tests.

test/built-ins/Temporal/PlainDate/prototype/with/leap-year.js

@@ -0,0 +1,24 @@
+// Copyright (C) 2025 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.prototype.with
+description: Check constraining days when year changes
+features: [Temporal]
+includes: [temporalHelpers.js]
+---*/
+
+const leapDay = new Temporal.PlainDate(2016, 2, 29);
+const options = { overflow: "reject" };
+
+TemporalHelpers.assertPlainDate(
+  leapDay.with({ year: 2012 }, options),
+  2012, 2, "M02", 29, "day not constrained when moving to another leap year");
+
+TemporalHelpers.assertPlainDate(
+  leapDay.with({ year: 2018 }),
+  2018, 2, "M02", 28, "day constrained when moving to a common year");
+
+assert.throws(RangeError, function () {
+  leapDay.with({ year: 2018 }, options);
+}, "reject when moving to a common year");