GH-10309: Don't register bean for internal GatewayMH.gatewayProxyFactoryBean (#10311)

Fixes: #10309

The call of the `BeanFactory.initializeBean()` on the internal `GatewayProxyFactoryBean`
instance of the `GatewayMessageHandler` causes an extra bean to be processed everywhere.
Since such a registration happens from the `GatewayMessageHandler.start()`,
the `SpringIntegrationTestExecutionListener#prepareTestInstance` may suffer from a `ConcurrentModificationException`
on the `autoStartupCandidates`.
Just because its logic is to call `start()` from that `autoStartupCandidates` collection iteration.

Essentially, we don't need that since a `GatewayMessageHandler` is a bean itself.
The `GatewayProxyFactoryBean` is used internally to avoid logic duplication of the proxy
creating on the provided interface.

* Rework the logic of the `GatewayMessageHandler` to call all the respective `BeanFactory`
call-backs manually.
* Add an `IntegrationFlow` bean with a `gateway()` into the `MockMessageHandlerTests`
to ensure that `SpringIntegrationTestExecutionListener` does not suffer from a `ConcurrentModificationException`
anymore.

**Auto-cherry-pick to `6.5.x`**

Author: artembilan

spring-integration-core/src/main/java/org/springframework/integration/gateway/GatewayMessageHandler.java

@@ -25,7 +25,6 @@
 import org.jspecify.annotations.Nullable;
 
 import org.springframework.beans.factory.BeanCreationException;
-import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
 import org.springframework.core.task.SimpleAsyncTaskExecutor;
 import org.springframework.integration.handler.AbstractReplyProducingMessageHandler;
 import org.springframework.integration.support.management.ManageableLifecycle;
@@ -158,9 +157,12 @@ private void initialize() {
 			this.gatewayProxyFactoryBean.setDefaultReplyTimeout(this.replyTimeout);
 		}
 
-		if (getBeanFactory() instanceof ConfigurableListableBeanFactory configurableListableBeanFactory) {
-			configurableListableBeanFactory.initializeBean(this.gatewayProxyFactoryBean, getComponentName() + "#gpfb");
-		}
+		this.gatewayProxyFactoryBean.setBeanName(getComponentName() + "#gpfb");
+		this.gatewayProxyFactoryBean.setBeanFactory(getBeanFactory());
+		this.gatewayProxyFactoryBean.setApplicationContext(getApplicationContext());
+		this.gatewayProxyFactoryBean.setBeanClassLoader(getBeanClassLoader());
+		this.gatewayProxyFactoryBean.afterPropertiesSet();
+
 		try {
 			this.exchanger = this.gatewayProxyFactoryBean.getObject();
 		}

spring-integration-test/src/test/java/org/springframework/integration/test/mock/MockMessageHandlerTests.java

@@ -36,6 +36,7 @@
 import org.springframework.integration.channel.DirectChannel;
 import org.springframework.integration.channel.QueueChannel;
 import org.springframework.integration.config.EnableIntegration;
+import org.springframework.integration.dsl.IntegrationFlow;
 import org.springframework.integration.endpoint.ReactiveStreamsConsumer;
 import org.springframework.integration.expression.ValueExpression;
 import org.springframework.integration.handler.ExpressionEvaluatingMessageHandler;
@@ -365,6 +366,12 @@ public ReactiveMessageHandler reactiveMessageHandler() {
 			return message -> Mono.empty();
 		}
 
+		// Before the fix for https://github.com/spring-projects/spring-integration/issues/10309 this test suite has failed
+		@Bean
+		IntegrationFlow flowWithGateway() {
+			return flow -> flow.gateway(subFlow -> subFlow.bridge());
+		}
+
 	}
 
 }