fix: emit full seconds in Instant and ZonedDateTime toISOString (RFC 3339)

Instant::toISOString() and ZonedDateTime::toISOString() emitted a compact
time that dropped zero seconds (e.g. 2023-11-14T22:14Z), which is valid
ISO 8601 but not valid RFC 3339. Section 5.6 of RFC 3339 requires the full
HH:MM:SS time even when the seconds are zero. These types are meant for
interoperability, so they must always emit RFC 3339 compliant output.

Build the time part of ZonedDateTime::toISOString() explicitly with a
mandatory seconds component instead of delegating to LocalTime's compact
formatter. Instant delegates to ZonedDateTime, so it is fixed transitively;
jsonSerialize() and __toString() route through toISOString(). LocalTime's
documented compact format is intentional and left unchanged.

This changes the serialized string representation and is therefore a BC
break; fixtures that pinned the compact output are updated accordingly.

src/ZonedDateTime.php

@@ -735,11 +735,31 @@ final readonly class ZonedDateTime implements JsonSerializable, Stringable
     /**
      * Returns the ISO 8601 representation of this zoned date time.
      *
+     * Unlike {@see LocalTime::toISOString()}, the time part always includes the
+     * seconds, so that the output is a valid RFC 3339 date-time. This type is
+     * primarily meant for interoperability, and RFC 3339 section 5.6 requires
+     * the full `HH:MM:SS` time even when the seconds are zero.
+     *
      * @return non-empty-string
      */
     public function toISOString(): string
     {
-        $string = $this->localDateTime . $this->timeZoneOffset;
+        $time = $this->localDateTime->getTime();
+
+        $hour = $time->getHour();
+        $minute = $time->getMinute();
+        $second = $time->getSecond();
+        $nano = $time->getNano();
+
+        $string = $this->localDateTime->getDate()
+            . 'T'
+            . ($hour < 10 ? '0' . $hour : $hour)
+            . ':'
+            . ($minute < 10 ? '0' . $minute : $minute)
+            . ':'
+            . ($second < 10 ? '0' . $second : $second)
+            . ($nano !== 0 ? '.' . rtrim(str_pad((string) $nano, 9, '0', STR_PAD_LEFT), '0') : '')
+            . $this->timeZoneOffset;
 
         if ($this->timeZone instanceof TimeZoneRegion) {
             $string .= '[' . $this->timeZone . ']';