Add #[NoDiscard] to CarbonImmutable modify(), next(), previous() and __call()

On PHP 8.5, DateTimeImmutable::modify() warns when its result is
discarded, but CarbonImmutable overrides it and lost the attribute.
Override modify(), next(), previous() and __call() in CarbonImmutable
with #[NoDiscard] so a discarded result warns again. The attribute is
ignored on older PHP versions, and mutable Carbon is unchanged.

Existing immutable tests that discarded results now keep them; in
IsTest::testIsSameMonth this was a real slip, the loop never moved $dt.

Fixes #3377

tests/CarbonImmutable/NoDiscardTest.php

@@ -0,0 +1,62 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * This file is part of the Carbon package.
+ *
+ * (c) Brian Nesbitt <brian@nesbot.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Tests\CarbonImmutable;
+
+use Carbon\Carbon as MutableCarbon;
+use Carbon\CarbonImmutable as Carbon;
+use ErrorException;
+use Generator;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\Attributes\RequiresPhp;
+use Tests\AbstractTestCase;
+
+#[RequiresPhp('>=8.5.0')]
+class NoDiscardTest extends AbstractTestCase
+{
+    public static function dataForDiscardedCalls(): Generator
+    {
+        yield ['modify', ['+1 day']];
+        yield ['next', []];
+        yield ['previous', []];
+        yield ['addDays', [2]];
+    }
+
+    #[DataProvider('dataForDiscardedCalls')]
+    public function testDiscardedResultTriggersWarning(string $method, array $arguments): void
+    {
+        $this->expectException(ErrorException::class);
+        $this->expectExceptionMessage(
+            "The return value of method Carbon\\CarbonImmutable::$method() should either be used or intentionally ignored by casting it as (void)",
+        );
+
+        $date = Carbon::parse('2026-10-02 12:00');
+
+        $this->withErrorAsException(static function () use ($date, $method, $arguments) {
+            $date->$method(...$arguments);
+        });
+    }
+
+    public function testMutableResultCanBeDiscarded(): void
+    {
+        $date = MutableCarbon::parse('2026-10-02 12:00');
+
+        $this->withErrorAsException(static function () use ($date) {
+            $date->modify('+1 day');
+            $date->next();
+            $date->addDays(2);
+        });
+
+        $this->assertSame('2026-10-12 00:00:00', $date->format('Y-m-d H:i:s'));
+    }
+}