fix: make RequestSchedulerInterface a singleton

RequestSchedulerInterface was not registered as singleton.
Engine and RetryMiddleware received different scheduler instances.
This resulted in separate queues instead of a shared one.

Author: teandr

src/Core/DefaultContainer.php

@@ -72,7 +72,7 @@ private function registerDefaultBindings(): void
             EventDispatcher::class,
         );
         $this->container->add(ClockInterface::class, SystemClock::class);
-        $this->container->add(
+        $this->container->addShared(
             RequestSchedulerInterface::class,
             /** @phpstan-ignore return.type */
             fn (): RequestSchedulerInterface => $this->container->get(ArrayRequestScheduler::class),