[13.x] Support #[Delay] attribute on queued mailables

[sumaiazaman]

Summary

The #[Delay] attribute is supported for queued jobs (Bus\Dispatcher), event listeners (Events\Dispatcher), and notifications (NotificationSender), but queued mailables only check the $delay property and ignore the attribute entirely.

This means the following does not work as expected:

use Illuminate\Queue\Attributes\Delay;

#[Delay(30)]
class WelcomeEmail extends Mailable implements ShouldQueue
{
    // The #[Delay] attribute is ignored — mailable is queued immediately
}

Changes

• Added ReadsClassAttributes support trait to Mailable
• Updated Mailable::queue() to use getAttributeValue() for reading delay, consistent with how Bus\Dispatcher::pushCommandToQueue(), Events\Dispatcher::queueHandler(), and NotificationSender::queueNotification() handle the #[Delay] attribute
• The $delay property still takes precedence when explicitly set (not default), matching the behavior of getAttributeValue()

Follows up on #59577 with the suggested change to use the support trait.

Test Plan

• Added test to verify #[Delay] attribute is respected on queued mailables
• Added test to verify property override still takes precedence over attribute
• Existing queued mailable tests still pass