GH-9709: Fix IntegrationFlow for input channel resolution

Fixes: #9709
Issue link: #9709

The Java DSL loses an `inputChannel` when existing channel is referenced by its name.
Then the target IntegrationFlow does not contain a bean reference for this channel and when we call its `getInputChannel()` we got something from middle of the flow.
However, that `inputChannel` is really expected to be an input for the flow.

* Fix `IntegrationFlowBeanPostProcessor` to get a bean by `MessageChannelReference` and populate it into `targetIntegrationComponents`
as we do for newly created `DirectChannel` if there is no bean for provided `MessageChannelReference`

**Auto-cherry-pick to `6.3.x`**

spring-integration-core/src/main/java/org/springframework/integration/dsl/context/IntegrationFlowBeanPostProcessor.java

@@ -75,6 +75,7 @@
 import org.springframework.integration.gateway.AnnotationGatewayProxyFactoryBean;
 import org.springframework.integration.support.context.NamedComponent;
 import org.springframework.lang.Nullable;
+import org.springframework.messaging.MessageChannel;
 import org.springframework.messaging.MessageHandler;
 import org.springframework.util.Assert;
 import org.springframework.util.CollectionUtils;
@@ -192,12 +193,16 @@ else if (useFlowIdAsPrefix) {
 			}
 			else if (component instanceof MessageChannelReference messageChannelReference) {
 				String channelBeanName = messageChannelReference.name();
+				MessageChannel channelByName;
 				if (!this.beanFactory.containsBean(channelBeanName)) {
-					DirectChannel directChannel = new DirectChannel();
-					registerComponent(registerBeanDefinitions, directChannel, channelBeanName,
+					channelByName = new DirectChannel();
+					registerComponent(registerBeanDefinitions, channelByName, channelBeanName,
 							beanSource, beanDescription, flowBeanName);
-					targetIntegrationComponents.put(directChannel, channelBeanName);
 				}
+				else {
+					channelByName = this.beanFactory.getBean(channelBeanName, MessageChannel.class);
+				}
+				targetIntegrationComponents.put(channelByName, channelBeanName);
 			}
 			else if (component instanceof SourcePollingChannelAdapterSpec spec) {
 				Map<Object, String> componentsToRegister = spec.getComponentsToRegister();

spring-integration-core/src/test/java/org/springframework/integration/dsl/composition/IntegrationFlowCompositionTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021-2023 the original author or authors.
+ * Copyright 2021-2024 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -202,11 +202,17 @@ IntegrationFlow middleFlow(IntegrationFlow firstFlow, IntegrationFlow lastFlow)
 					.to(lastFlow);
 		}
 
+		@Bean
+		DirectChannel lastFlowInput() {
+			return new DirectChannel();
+		}
+
 		@Bean
 		IntegrationFlow lastFlow() {
-			return f -> f
+			return IntegrationFlow.from("lastFlowInput")
 					.<String, String>transform(p -> p + ", and last flow")
-					.channel(c -> c.queue("lastFlowResult"));
+					.channel(c -> c.queue("lastFlowResult"))
+					.get();
 		}
 
 	}