Add test to make sure that the fallback bean exists in the context

kafka/src/test/groovy/docs/producer/fallback/MessageClient.java

@@ -2,7 +2,9 @@
 
 import io.micronaut.configuration.kafka.annotation.KafkaClient;
 import io.micronaut.configuration.kafka.annotation.Topic;
+import io.micronaut.context.annotation.Requires;
 
+@Requires(property = "spec.name", value = "MessageClientFallbackSpec")
 @KafkaClient
 public interface MessageClient {
 

kafka/src/test/groovy/docs/producer/fallback/MessageClientFallback.java

@@ -7,6 +7,7 @@
 import jakarta.inject.Singleton;
 // end::imports[]
 
+@Requires(property = "spec.name", value = "MessageClientFallbackSpec")
 // tag::clazz[]
 @Requires(property = "kafka.enabled", notEquals = StringUtils.TRUE, defaultValue = StringUtils.TRUE) // <1>
 @Replaces(MessageClient.class) // <2>

kafka/src/test/groovy/docs/producer/fallback/MessageClientFallbackSpec.groovy

@@ -0,0 +1,30 @@
+package docs.producer.fallback
+
+import io.micronaut.context.ApplicationContext
+import io.micronaut.context.annotation.Property
+import io.micronaut.test.extensions.spock.annotation.MicronautTest
+import jakarta.inject.Inject
+import spock.lang.Specification
+
+@MicronautTest
+@Property(name = "spec.name", value = "MessageClientFallbackSpec")
+@Property(name = "kafka.enabled", value = "false")
+class MessageClientFallbackSpec extends Specification {
+
+    @Inject
+    ApplicationContext context
+
+    void "test that context contains the fallback bean"() {
+        when:
+        MessageClientFallback bean = context.getBean(MessageClientFallback)
+
+        then:
+        bean != null
+
+        when:
+        bean.send('message')
+
+        then:
+        thrown(UnsupportedOperationException)
+    }
+}