GH-107: Make Splitter Function as Flux-based

Fixes: https://github.com/spring-cloud/spring-functions-catalog/issues/107

When we have a composition like this:

```
spring.cloud.function.definition = fileSupplier|splitterFunction
```

Then final "function" signature is like this `Supplier<Flux<Message<List<Message<?>>>>>`.
And that is exactly what we don't expected from the splitter in the end of the composition.
While Spring Cloud Stream supports de-batching, it works for a `List` output only if function is bound by itself.
In case of composition we got just a `Supplier`.

* Rework `SplitterFunctionConfiguration` for `splitterFunction` from `Function<Message<?>, List<Message<?>>>`
to `Function<Flux<Message<?>>, Flux<Message<?>>>` signature to support every possible simple and composed bindings
in Spring Cloud Stream
* Rework `SplitterFunctionApplicationTests` for new expected `Function<Flux<Message<?>>, Flux<Message<?>>>` signature
* Rework `zip-split-rabbit-binder` sample to not use a `flattenFunction` workaround
and fully rely on whatever is new for the `splitterFunction`
* Fix `ZipSplitRabbitBinderApplicationTests` moving the `@RabbitListener` into a `@TestConfiguration`.
Apparently in a new Spring Boot version the test class is registered as a bean much later than normal application context startup.
Therefore, even if the `@RabbitListener` parsed and registered properly, the `RabbitAdmin` bean
has been already started to see our extra bean definition for the `@QueueBinding`

Changing signature for the splitterFunction to reactive types would make it working even with a Supplier composition.

Fix JDBC & MongoDB suppliers to deal with a new version of Splitter function

Fix Checkstyle violations

Use `IntegrationReactiveUtils.messageSourceToFlux()` API

The `IntegrationReactiveUtils.messageSourceToFlux()` provides convenient API to represent a `MessageSource`
as a `Flux` to poll this source.
The API has an error handling logic and delay when no data emitted by the source

* Remove `org.springframework.cloud` dependencies from the project
since we don't use `@PollableBean` anymore, which comes from the `spring-cloud-function-context`
* Simplify `JdbcSupplierConfiguration` and `MongodbSupplierConfiguration` code more: more injections to the respective bean method.
* Use `(__) ->` lambda syntax for unused argument
* Remove unused `ThreadLocalFluxSinkMessageChannel` internal class
* Update Copyrights of the classes in this change

Upgrade to Gradle `8.12`

function/spring-splitter-function/src/main/java/org/springframework/cloud/fn/splitter/SplitterFunctionConfiguration.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2011-2024 the original author or authors.
+ * Copyright 2011-2025 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.
@@ -17,11 +17,9 @@
 package org.springframework.cloud.fn.splitter;
 
 import java.nio.charset.Charset;
-import java.util.List;
 import java.util.Optional;
 import java.util.function.Function;
 
-import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
 
 import org.springframework.beans.factory.annotation.Qualifier;
@@ -32,13 +30,12 @@ import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Conditional;
-import org.springframework.integration.channel.ReactiveStreamsSubscribableChannel;
+import org.springframework.integration.channel.FluxMessageChannel;
 import org.springframework.integration.file.splitter.FileSplitter;
 import org.springframework.integration.splitter.AbstractMessageSplitter;
 import org.springframework.integration.splitter.DefaultMessageSplitter;
 import org.springframework.integration.splitter.ExpressionEvaluatingSplitter;
 import org.springframework.messaging.Message;
-import org.springframework.messaging.MessageChannel;
 
 /**
  * Auto-configuration for Splitter function.
@@ -51,7 +48,7 @@ import org.springframework.messaging.MessageChannel;
 public class SplitterFunctionConfiguration {
 
 	@Bean
-	public Function<Message<?>, List<Message<?>>> splitterFunction(
+	public Function<Flux<Message<?>>, Flux<Message<?>>> splitterFunction(
 			@Qualifier("expressionSplitter") Optional<AbstractMessageSplitter> expressionSplitter,
 			@Qualifier("fileSplitter") Optional<AbstractMessageSplitter> fileSplitter,
 			@Qualifier("defaultSplitter") Optional<AbstractMessageSplitter> defaultSplitter,
@@ -60,13 +57,13 @@ public class SplitterFunctionConfiguration {
 		AbstractMessageSplitter messageSplitter = expressionSplitter.or(() -> fileSplitter)
 			.or(() -> defaultSplitter)
 			.get();
+
 		messageSplitter.setApplySequence(splitterFunctionProperties.isApplySequence());
-		ThreadLocalFluxSinkMessageChannel outputChannel = new ThreadLocalFluxSinkMessageChannel();
+		FluxMessageChannel inputChannel = new FluxMessageChannel();
+		inputChannel.subscribe(messageSplitter);
+		FluxMessageChannel outputChannel = new FluxMessageChannel();
 		messageSplitter.setOutputChannel(outputChannel);
-		return (message) -> {
-			messageSplitter.handleMessage(message);
-			return outputChannel.publisherThreadLocal.get();
-		};
+		return (messageFlux) -> Flux.from(outputChannel).doOnRequest((__) -> inputChannel.subscribeTo(messageFlux));
 	}
 
 	@Bean
@@ -117,22 +114,4 @@ public class SplitterFunctionConfiguration {
 
 	}
 
-	private static final class ThreadLocalFluxSinkMessageChannel
-			implements MessageChannel, ReactiveStreamsSubscribableChannel {
-
-		private final ThreadLocal<List<Message<?>>> publisherThreadLocal = new ThreadLocal<>();
-
-		@Override
-		@SuppressWarnings("unchecked")
-		public void subscribeTo(Publisher<? extends Message<?>> publisher) {
-			this.publisherThreadLocal.set(Flux.from(publisher).collectList().cast(List.class).block());
-		}
-
-		@Override
-		public boolean send(Message<?> message, long l) {
-			throw new UnsupportedOperationException("This channel only supports a reactive 'subscribeTo()' ");
-		}
-
-	}
-
 }

function/spring-splitter-function/src/test/java/org/springframework/cloud/fn/splitter/SplitterFunctionApplicationTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2011-2024 the original author or authors.
+ * Copyright 2011-2025 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.
@@ -16,10 +16,12 @@
 
 package org.springframework.cloud.fn.splitter;
 
-import java.util.List;
+import java.time.Duration;
 import java.util.function.Function;
 
 import org.junit.jupiter.api.Test;
+import reactor.core.publisher.Flux;
+import reactor.test.StepVerifier;
 
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
@@ -28,19 +30,18 @@ import org.springframework.messaging.Message;
 import org.springframework.messaging.support.GenericMessage;
 import org.springframework.test.annotation.DirtiesContext;
 
-import static org.assertj.core.api.Assertions.assertThat;
-
 @SpringBootTest(properties = "splitter.expression=payload.split(',')")
 @DirtiesContext
 public class SplitterFunctionApplicationTests {
 
 	@Autowired
-	Function<Message<?>, List<Message<?>>> splitter;
+	Function<Flux<Message<?>>, Flux<Message<?>>> splitter;
 
 	@Test
 	public void testExpressionSplitter() {
-		List<Message<?>> messageList = this.splitter.apply(new GenericMessage<>("hello,world"));
-		assertThat(messageList).extracting((m) -> m.getPayload().toString()).contains("hello", "world");
+		Flux<Message<?>> messageFlux = this.splitter.apply(Flux.just(new GenericMessage<>("hello,world")));
+		Flux<String> payloads = messageFlux.map(Message::getPayload).map(Object::toString);
+		StepVerifier.create(payloads).expectNext("hello", "world").thenCancel().verify(Duration.ofSeconds(30));
 	}
 
 	@SpringBootApplication