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

[dualfroz]

Problem

ChronosTime misinterprets a time string whose fractional-seconds part has fewer
than six digits. The fractional part is treated as a literal microsecond count
instead of a fraction of a second:

(new Cake\Chronos\ChronosTime('12:00:00.5'))->format('H:i:s.u');
// "12:00:00.000005"  (5 microseconds)
// expected "12:00:00.500000"  (half a second, matching DateTime)

(new DateTime('12:00:00.5'))->format('u'); // "500000"

So 12:00:00.5 becomes 5 microseconds rather than 500000, diverging from PHP's
own DateTime and from the round-trip used when constructing a ChronosTime
from a DateTimeInterface (which formats to six digits).

Root cause

src/ChronosTime.php, parseString():

$microseconds = (int)substr($matches[4] ?? '', 0, 6);

$matches[4] is the digit run after the decimal point. Passing it straight to
(int) reads "5" as 5, ignoring that these are the most-significant digits of
a fractional second. Only six-digit input (for example .500000) happened to
work, which is why the issue was not caught: every existing test uses a full
six-digit fraction (or a seven-digit one that is truncated to six).

The fix

Right-pad the (at most six) fractional digits to microseconds before converting:

$microseconds = (int)str_pad(substr($matches[4] ?? '', 0, 6), 6, '0', STR_PAD_RIGHT);

• .5 -> 500000
• .05 -> 50000
• .500000 -> 500000 (unchanged)
• .000005 -> 5 (unchanged)
• .9999991-> truncated to 999999 (unchanged; existing behaviour preserved)

This matches how DateTime interprets fractional seconds.

Test

Added ChronosTimeTest::testConstructFromStringWithFractionalSeconds, asserting
that 12:00:00.5, 12:00:00.05 and 12:00:00.000005 produce
500000, 50000 and 5 microseconds respectively.