Temporal: Add coverage for weird PlainMonthDay leap day constrain case

If you have a leap day, such as February 29, and you get input such as
{ monthCode: "M02", day: 30 }, { overflow: "constrain" }, then you want
the day to be constrained to the leap day February 29, not February 28
as the maximum day would be in a common year. Add tests for this case
for each supported calendar.

Author: ptomato

test/built-ins/Temporal/PlainMonthDay/from/constrain-to-leap-day.js

@@ -0,0 +1,11 @@
+// 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.plainmonthday.from
+description: Properly constrain February 30 to February 29, not 28
+features: [Temporal]
+---*/
+
+const md = Temporal.PlainMonthDay.from({ monthCode: "M02", day: 30 }, { overflow: "constrain" });
+assert.sameValue(md.day, 29, "M02-30 should constrain to 29, not 28");

test/intl402/Temporal/PlainMonthDay/from/constrain-to-leap-day.js

@@ -0,0 +1,32 @@
+// 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.plainmonthday.from
+description: Properly constrain a day that is one past a leap day
+features: [Temporal]
+---*/
+
+const tests = [
+  ["buddhist", "M02", 30],
+  ["chinese", "M01", 31],
+  ["coptic", "M13", 7],
+  ["dangi", "M01", 31],
+  ["ethioaa", "M13", 7],
+  ["ethiopic", "M13", 7],
+  ["gregory", "M02", 30],
+  ["hebrew", "M02", 31],
+  ["indian", "M01", 32],
+  ["islamic-civil", "M01", 31],
+  ["islamic-tbla", "M01", 31],
+  ["islamic-umalqura", "M01", 31],
+  ["japanese", "M02", 30],
+  ["persian", "M12", 31],
+  ["roc", "M02", 30],
+];
+
+for (const [calendar, monthCode, day] of tests) {
+  const md = Temporal.PlainMonthDay.from({ calendar, monthCode, day }, { overflow: "constrain" });
+  assert.sameValue(md.day, day - 1,
+    `${calendar}: ${monthCode}-${day} should constrain to ${day - 1}, not ${day - 2}`)
+}