The `AbstractMethodAnnotationPostProcessor` makes a decision to use a `MessageHandler`
bean definition as is without wrapping by the type of `MessageHandler`.
There are some of them can be configured via `AbstractSimpleMessageHandlerFactoryBean`,
e.g. an `AggregatorFactoryBean`
* Check for the `AbstractSimpleMessageHandlerFactoryBean` to return as is
* Widen the replying producer by the `AbstractMessageProducingHandler` type
* Add an `AggregatorFactoryBean` with a `@ServiceActivator` configuration to test coverage
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