[Temporal] Add coverage for month before earliest supported month

For PlainYearMonth.from with non-ISO calendars, add test coverage
for the cases where the month is one month before the earliest supported
month, and the month is one month after the latest supported month
(an exception should be thrown).

Author: catamorphism

test/intl402/Temporal/PlainYearMonth/from/extreme-unsupported-dates.js

@@ -0,0 +1,52 @@
+// Copyright (C) 2026 Igalia, S.L. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+
+/*---
+esid: sec-temporal.plaindate.from
+description: from() throws for month before earlier supported month, and month after latest supported month
+features: [Temporal, Intl.Era-monthcode]
+---*/
+
+// Lunisolar/lunar calendars can't accurately predict celestial orbits for dates
+// far into the past/future. Skip `chinese` and `dangi`. `islamic-umalqura` is
+// okay because it is specified to fall back to `islamic-civil` outside the
+// range of accuracy.
+
+// Note that the earliest PlainYearMonth that can be constructed in a calendar
+// is the earliest month whose first day occurs after ISO -271821-04-19
+
+const testData = [
+  ["buddhist", -271278, 3, "M03", "be", -271278, 276303, 10, "M10", "be", 276303],
+  ["coptic", -272099, 3, "M03", "am", -272099, 275471, 6, "M06", "am", 275471],
+  ["ethioaa", -266323, 3, "M03", "aa", -266323, 281247, 6, "M06", "aa", 281247],
+  ["ethiopic", -271823, 3, "M03", "aa", -266323, 275747, 6, "M06", "am", 275747],
+  ["gregory", -271821, 3, "M03", "bce", 271822, 275760, 10, "M10", "ce", 275760],
+  ["hebrew", -268058, 11, "M11", "am", -268058, 279517, 11, "M11", "am", 279517],
+  ["indian", -271899, 1, "M01", "shaka", -271899, 275682, 7, "M07", "shaka", 275682],
+  ["islamic-civil", -280804, 3, "M03", "bh", 280805, 283583, 6, "M06", "ah", 283583],
+  ["islamic-tbla", -280804, 3, "M03", "bh", 280805, 283583, 6, "M06", "ah", 283583],
+  ["islamic-umalqura", -280804, 3, "M03", "bh", 280805, 283583, 6, "M06", "ah", 283583],
+  ["japanese", -271821, 3, "M03", "bce", 271822, 275760, 10, "M10", "reiwa", 273742],
+  ["persian", -272442, 1, "M01", "ap", -272442, 275139, 8, "M08", "ap", 275139],
+  ["roc", -273732, 3, "M03", "broc", 273733, 273849, 10, "M10", "roc", 273849],
+];
+
+for (const [calendar, minYear, minMonth, minMonthCode, minEra, minEraYear, maxYear, maxMonth, maxMonthCode, maxEra, maxEraYear] of testData) {
+  assert.throws(RangeError, () => Temporal.PlainYearMonth.from({
+    calendar,
+    year: minYear,
+    era: minEra,
+    eraYear: minEraYear,
+    month: minMonth,
+    monthCode: minMonthCode,
+  }), `calendar ${calendar}: month before min month should throw`);
+
+  assert.throws(RangeError, () => Temporal.PlainYearMonth.from({
+    calendar,
+    year: maxYear,
+    era: maxEra,
+    eraYear: maxEraYear,
+    month: maxMonth,
+    monthCode: maxMonthCode,
+  }), `calendar ${calendar}: month after max month should throw`);
+}