fix: parse fractional seconds in ChronosTime as a fraction of a second

ChronosTime::parseString() passed the fractional-seconds digits straight to
(int), so a short fraction such as .5 was read as 5 microseconds instead of
500000 (half a second), diverging from DateTime. Right-pad the fractional digits
to microseconds before converting.

src/ChronosTime.php

@@ -122,7 +122,9 @@ class ChronosTime implements Stringable
         $hours = (int)$matches[1];
         $minutes = (int)$matches[2];
         $seconds = (int)($matches[3] ?? 0);
-        $microseconds = (int)substr($matches[4] ?? '', 0, 6);
+        // The fraction is of a second, so pad on the right to microseconds:
+        // without it ".5" reads as 5us instead of 500000 (half a second).
+        $microseconds = (int)str_pad(substr($matches[4] ?? '', 0, 6), 6, '0', STR_PAD_RIGHT);
 
         if ($hours > 24 || $minutes > 59 || $seconds > 59 || $microseconds > 999_999) {
             throw new InvalidArgumentException(sprintf('Time string `%s` contains invalid values.', $time));

tests/TestCase/ChronosTimeTest.php

@@ -53,6 +53,19 @@ class ChronosTimeTest extends TestCase
         $this->assertSame('00:59:59.999999', $t->format('H:i:s.u'));
     }
 
+    public function testConstructFromStringWithFractionalSeconds(): void
+    {
+        // The fractional part is a fraction of a second, matching DateTime.
+        $t = new ChronosTime('12:00:00.5');
+        $this->assertSame('12:00:00.500000', $t->format('H:i:s.u'));
+
+        $t = new ChronosTime('12:00:00.05');
+        $this->assertSame('12:00:00.050000', $t->format('H:i:s.u'));
+
+        $t = new ChronosTime('12:00:00.000005');
+        $this->assertSame('12:00:00.000005', $t->format('H:i:s.u'));
+    }
+
     public function testConstructFromInstance(): void
     {
         $t = new ChronosTime(new DateTimeImmutable('23:59:59.999999'));