GH-3912: Handler: ignore Groovy generated methods

Fixes https://github.com/spring-projects/spring-integration/issues/3912

**Cherry-pick to `5.5.x`**

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

@@ -16,7 +16,7 @@
 
 package org.springframework.integration.groovy.dsl.test
 
-
+import groovy.transform.CompileStatic
 import org.junit.jupiter.api.Test
 import org.springframework.beans.factory.BeanFactory
 import org.springframework.beans.factory.annotation.Autowired
@@ -45,6 +45,7 @@ import reactor.core.publisher.Flux
 import reactor.test.StepVerifier
 
 import java.time.Duration
+import java.util.concurrent.atomic.AtomicReference
 import java.util.function.Function
 
 import static org.springframework.integration.groovy.dsl.IntegrationGroovyDsl.integrationFlow
@@ -196,6 +197,19 @@ class GroovyDslTests {
 		assert this.wireTapChannel.receive(10_000)?.payload == 'test'
 	}
 
+	@Autowired
+	@Qualifier('externalServiceFlow.input')
+	private MessageChannel externalServiceFlowInput
+
+	@Autowired
+	GroovyTestService groovyTestService
+
+	@Test
+	void 'handle service'() {
+		this.externalServiceFlowInput.send(new GenericMessage<Object>('test'))
+		assert groovyTestService.result.get() == 'TEST'
+	}
+
 	@Configuration
 	@EnableIntegration
 	static class Config {
@@ -301,6 +315,30 @@ class GroovyDslTests {
 			}
 		}
 
+		@Bean
+		myService() {
+			new GroovyTestService()
+		}
+
+		@Bean
+		externalServiceFlow(GroovyTestService groovyTestService) {
+			integrationFlow {
+				handle groovyTestService
+			}
+
+		}
+
+	}
+
+	@CompileStatic
+	static class GroovyTestService {
+
+		AtomicReference<String> result = new AtomicReference<>()
+
+		void handlePayload(String payload) {
+			result.set payload.toUpperCase()
+		}
+
 	}
 
 }