Block for externalization in outbox mode

[rolandbeisel]

Fix synchronous handling for Outbox event externalization across transport configurations

Summary

This PR ensures that Outbox event externalization is handled synchronously across all supported messaging transport modules (AMQP, JMS, Kafka, and Spring Messaging). The Outbox handler implementations now block until the externalization is complete and properly propagate exceptions.

Details

• Modules affected:

  • spring-modulith-events-amqp
  • spring-modulith-events-jms
  • spring-modulith-events-kafka
  • spring-modulith-events-messaging

• What was changed:

  • The Outbox handler in each transport configuration now calls .get() on the returned CompletableFuture to ensure synchronous execution.
  • Exceptions (InterruptedException, ExecutionException) are caught and rethrown as RuntimeException, with the thread interrupt status restored as appropriate.
  • This guarantees that failures during event externalization are not silently ignored and are surfaced to the caller.

Why

• To provide consistent, reliable, and predictable event externalization semantics across all messaging modules.
• To ensure that errors during outbox processing are not lost and can be handled by the application.

[odrotbohm]

Tweaked as discussed 1:1.