[Messenger] Drop the messages queued by a failed nested dispatch - #66032

A message dispatched with a DispatchAfterCurrentBusStamp from inside a handler is queued and dispatched once the root dispatch is over. When the nested dispatch that queued it fails, the queued messages are dispatched anyway, although the work that queued them never completed.

The root dispatch already drops its queue when handling throws, with the reasoning that the queued messages were likely dependent on the work that queued them. The nested path did not, so the two halves of the same middleware disagreed.

public function __invoke(RegisterUser $command): void
{
    try {
        // this dispatch queues UserRegistered, then throws
        $this->bus->dispatch(new CreateProfile($command->getUuid()));
    } catch (\Throwable) {
        // the caller decides to carry on
    }

    // UserRegistered was dispatched anyway, for a profile that was never created
}

Found while working on #65902, which turns each attempt of a synchronous retry into a nested dispatch and therefore meets this on every failed attempt. It is a bug of its own, so it is proposed here separately for the lowest maintained branch.

Checks

• ./phpunit src/Symfony/Component/Messenger/Tests on 6.4: 425 tests, green.
• testMessagesQueuedByAFailedNestedDispatchAreDropped errors without the fix, because the event queued by the failed nested dispatch reaches the handling middleware a third time.