This PR improves the withinSessionLevelLock() API by invoking the callback only when the lock is successfully acquired, eliminating the need for manual wasAcquired checks inside the callback.

Before

$result = $locker->withinSessionLevelLock($conn, $key, function ($handle) {
    if (!$handle->wasAcquired) {
        return null; // Consumer must check inside callback
    }
    return processOrder();
}, $timeout);

### After
```php
$handle = $locker->withinSessionLevelLock($conn, $key, fn() => processOrder(), $timeout);

if ($handle->wasAcquired) {
    echo $handle->result; // Result of callback
} else {
    // Lock not acquired, callback was not invoked
}

Key improvements

1. Callback only on success — The callback is invoked only when the lock is acquired. If the lock is held by another process, the callback is skipped and wasAcquired = false.

2. Cleaner API — Consumer checks wasAcquired outside the callback, not inside. The callback can focus solely on the critical section logic.

3. New return type — Returns WithinSessionLevelLockHandle with:

   • wasAcquired: bool — Whether the lock was acquired
   • result: mixed — Result of the callback (null if lock not acquired)

4. Simpler callback signature — Callback no longer receives SessionLevelLockHandle argument, since it's only invoked when the lock is held.