Add nullChannel() to Groovy DSL

The Groovy DSL was missing support for the `nullChannel()`, which is a
special channel that discards messages (acts like `/dev/null`). This
prevents users from using `nullChannel` in their Groovy integration
flows.

Add unit tests to verify:
- `nullChannel` can be used at the end of a flow
- `nullChannel` can be used after transform operations
- Messages are properly discarded without exceptions

* Refactor `nullChannel` tests to use bean configuration

The original implementation had left the beans as dead code after initial tests were deemed to weak.
However, the dynamic flow increased the amount of code to maintain.

Changes include:
- Replace dynamic flow creation with `@Bean` configuration methods
- Add shared `nullCheckWireTapChannel` bean for message verification
- Simplify test methods by using autowired channels
- Add `@since 7.0.1` annotation to `nullChannel()` method documentation

* Format NullChannel DSL javadoc

NullChannel for groovy DSL `see` and `since` annotation were moved to correct location

Author: cppwfs

spring-integration-groovy/src/main/groovy/org/springframework/integration/groovy/dsl/GroovyIntegrationFlowDefinition.groovy

@@ -103,6 +103,18 @@ class GroovyIntegrationFlowDefinition {
 		this
 	}
 
+	/**
+	 * Populate a {@link org.springframework.integration.channel.NullChannel} instance
+	 * at the current {@link IntegrationFlow} chain position.
+	 * The nullChannel acts like "/dev/null".
+	 * @since 7.0.1
+	 * @see org.springframework.integration.channel.NullChannel
+	 */
+	GroovyIntegrationFlowDefinition nullChannel() {
+		this.delegate.nullChannel()
+		this
+	}
+
 	/**
 	 * Populate a {@link org.springframework.integration.dsl.support.MessageChannelReference} instance
 	 * at the current {@link IntegrationFlow} chain position.

spring-integration-groovy/src/test/groovy/org/springframework/integration/groovy/dsl/test/GroovyDslTests.groovy

@@ -220,6 +220,35 @@ class GroovyDslTests {
 		assert groovyTestService.result.get() == 'TEST'
 	}
 
+	@Autowired
+	@Qualifier('nullChannelFlow.input')
+	private MessageChannel nullChannelFlowInput
+
+	@Autowired
+	private MessageChannel nullCheckWireTapChannel
+
+
+	@Test
+	void 'nullChannelFlow discards messages'() {
+		nullChannelFlowInput.send MessageBuilder.withPayload('test').build()
+		def tappedMessage = nullCheckWireTapChannel.receive(1000)
+
+		//verify that a message was sent and assume nullChannel discarded it.
+		assert tappedMessage?.payload == 'test'
+	}
+
+	@Autowired
+	@Qualifier('nullChannelWithTransformFlow.input')
+	private MessageChannel nullChannelWithTransformFlowInput
+
+	@Test
+	void 'nullChannel can be used after transform in a flow'() {
+		nullChannelWithTransformFlowInput.send MessageBuilder.withPayload('test').build()
+		def tappedMessage = nullCheckWireTapChannel.receive(1000)
+		//verify that a message was sent, transformed and assume nullChannel discarded it.
+		assert tappedMessage?.payload == 'TEST'
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@EnableIntegration
 	static class Config {
@@ -361,6 +390,30 @@ class GroovyDslTests {
 
 		}
 
+		@Bean
+		nullChannelFlow(@Qualifier('nullCheckWireTapChannel') MessageChannel nullCheckWireTapChannel) {
+			integrationFlow {
+				wireTap(nullCheckWireTapChannel)
+				nullChannel()
+			}
+		}
+
+		@Bean
+		nullChannelWithTransformFlow(@Qualifier('nullCheckWireTapChannel') MessageChannel nullCheckWireTapChannel) {
+			integrationFlow {
+				transform {
+					transformer { it.toUpperCase() }
+				}
+				wireTap(nullCheckWireTapChannel)
+				nullChannel()
+			}
+		}
+
+		@Bean
+		MessageChannel nullCheckWireTapChannel() {
+			new QueueChannel()
+		}
+
 	}
 
 	@CompileStatic