[11.x] Fix withoutOverlapping via PendingEventAttributes proxy

[istiak-tridip]

This PR addresses a bug with the withoutOverlapping method when used through the PendingEventAttributes proxy.

Example Code:

Schedule::runInBackground()
    ->withoutOverlapping()
    ->everyMinute()
    ->command('inspire');

The above throws the following exception:

Undefined property: Illuminate\Console\Scheduling\PendingEventAttributes::$mutex, vendor/laravel/framework/src/Illuminate/Console/Scheduling/ManagesAttributes.php, 145

This issue occurs during schedule execution, not at Event instance creation, so existing tests didn’t catch it.

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Thanks to @decadence for reporting this bug.

[decadence]

Yes. With this fix Schedule doesn't throw Exception 👍