fix: keep intervals built by createFromDateString() when deep copying

Since PHP 8.2 a DateInterval created by DateInterval::createFromDateString()
exposes only "from_string" and "date_string" instead of the y/m/d/h/i/s
fields, and "from_string" is ignored when assigned to. Rebuilding such an
interval by copying its visible properties onto a new DateInterval therefore
produced a zero interval, so a copied interval silently stopped shifting
dates.

Read the properties once and rebuild a date-string interval from the string
it was created from, leaving the existing property copy for every other
interval. Cloning would also fix it but would invoke __clone() on userland
DateInterval subclasses, which #76 explicitly rules out.

tests/DeepCopyTest/TypeFilter/Date/DateIntervalFilterTest.php

@@ -3,6 +3,7 @@
 namespace DeepCopyTest\TypeFilter\Date;
 
 use DateInterval;
+use DateTimeImmutable;
 use DeepCopy\TypeFilter\Date\DateIntervalFilter;
 use PHPUnit\Framework\TestCase;
 
@@ -22,4 +23,45 @@ class DateIntervalFilterTest extends TestCase
         $this->assertEquals($object, $copy);
         $this->assertNotSame($object, $copy);
     }
+
+    public function test_it_deep_copies_a_DateInterval_created_from_a_date_string()
+    {
+        $object = DateInterval::createFromDateString('2 days');
+
+        $filter = new DateIntervalFilter();
+
+        $copy = $filter->apply($object);
+
+        // The copy has to shift a date by the same amount as the original.
+        $base = new DateTimeImmutable('2026-01-01 00:00:00');
+
+        $this->assertSame(
+            $base->add($object)->format('Y-m-d H:i:s'),
+            $base->add($copy)->format('Y-m-d H:i:s')
+        );
+
+        $this->assertEquals($object, $copy);
+        $this->assertNotSame($object, $copy);
+    }
+
+    public function test_it_deep_copies_an_inverted_DateInterval_with_microseconds()
+    {
+        $object = new DateInterval('PT1S');
+        $object->invert = 1;
+        $object->f = 0.25;
+
+        $filter = new DateIntervalFilter();
+
+        $copy = $filter->apply($object);
+
+        $this->assertEquals($object, $copy);
+        $this->assertNotSame($object, $copy);
+
+        $base = new DateTimeImmutable('2026-01-01 00:00:00');
+
+        $this->assertSame(
+            $base->add($object)->format('Y-m-d H:i:s.u'),
+            $base->add($copy)->format('Y-m-d H:i:s.u')
+        );
+    }
 }