[2.x] feat(queue): restore per-class queue routing on AbstractJob

[imorland]

Summary

Restores the $onQueue static property + constructor that 1.x's AbstractJob exposed, and wires every in-tree job through it so operators can route any shipped job onto a dedicated queue without subclassing.

Without this, there's no built-in way for an operator (or an extension) to dispatch all instances of a given job class onto a dedicated queue without patching every dispatch site. Multiple 2.x-compatible extensions have already re-implemented this hook themselves — see issue #4653 for examples from FoF\GeoIP\Jobs\RetrieveIP and Flarum\Gdpr\Jobs\GdprJob. Both have settled on the name $onQueue (rather than 1.x's $sendOnQueue), so this PR adopts the same name.

Changes

Flarum\Queue\AbstractJob (commit 1):

• New public static ?string $onQueue = null; property.
• New __construct() that calls $this->onQueue(static::$onQueue) when the static property is set on the concrete subclass (late-static-bound, so each subclass gets its own routing knob).

Every in-tree job that has its own constructor (commit 2) now calls parent::__construct() so it inherits the routing behavior:

• Flarum\Notification\Job\SendEmailNotificationJob
• Flarum\Notification\Job\SendNotificationsJob
• Flarum\Mail\Job\SendAbandonedExtensionsEmailJob
• Flarum\Mail\Job\SendInformationalEmailJob
• Flarum\User\Job\RequestPasswordResetJob
• Flarum\Search\Job\IndexJob
• Flarum\Messages\Job\SendMessageNotificationsJob
• Flarum\Pusher\SendPusherNotificationsJob
• Flarum\ExtensionManager\Job\ComposerCommandJob
• Flarum\Mentions\Job\SendMentionsNotificationsJob
• Flarum\Gdpr\Jobs\ExportJob, ErasureJob

Cleanups for jobs that previously implemented the hook themselves:

• Flarum\Gdpr\Jobs\GdprJob: drop the local $onQueue shadow (inherited now).
• Flarum\Realtime\Push\Jobs\Job: drop the local $onQueue shadow and the workaround __construct() (inherited now). All concrete Send*Job subclasses in the same directory already call parent::__construct(), so routing flows through unchanged.

Behavior

• AbstractJob::$onQueue defaults to null, so default behavior is unchanged.
• Operators (or extensions) set SomeJob::$onQueue = 'queue-name' once at boot to route all instances of that job class onto a specific queue.
• Late-static-binding means a Child extends Parent subclass's own $onQueue value wins over the parent's, as expected.

Test plan

• New unit test tests/unit/Queue/AbstractJobTest.php covers: default behavior (no routing), routing via the static property, and late-static-binding under inheritance.
• Existing queue unit + integration tests pass unchanged (tests/unit/Queue, tests/integration/queue).
• PHPStan clean on every changed file.

Closes #4653