This PR fixes a behavioral inconsistency between NativeClock and MockClock when handling negative sleep durations.

The Problem:

• NativeClock::sleep(-10): Silently does nothing. The time does not change.
• MockClock::sleep(-10): Travels backward in time by 10 seconds.

This inconsistency can lead to unreliable tests, as the mock does not accurately represent the real implementation's behavior for this edge case.

The Fix:
This fix adds a guard clause to MockClock::sleep() to ignore any duration less than or equal to zero, perfectly matching the behavior of NativeClock.

Before (The Bug):

$clock = new MockClock('2000-01-01 12:00:00');
$clock->sleep(-10);
// $clock->now() is '2000-01-01 11:59:50'

After (The Fix):

$clock = new MockClock('2000-01-01 12:00:00');
$clock->sleep(-10);
// $clock->now() is '2000-01-01 12:00:00'