GH-9866: Fix FluxMessageChannel for race condition on destroy

Fixes: #9866
Issue link: #9866

Due to race condition between `sink.tryEmitNext()` and `sink.emitComplete()``,
there could be a situation when `onNext` signal slips after `onComplete`.
Appears on fast producers into this `FluxMessageChannel`.
That leads to error like `Spec. Rule 1.3 - onSubscribe, onNext, onError and onComplete signaled to a Subscriber MUST be signaled serially.`

* Fix `FluxMessageChannel` to check for `this.active` one more time just before `sink.tryEmitNext()` call
* Also mitigate a race condition with a `Sinks.EmitFailureHandler.busyLooping(Duration.ofSeconds(1))` in the `destroy()`
instead of `Sinks.EmitFailureHandler.FAIL_FAST`.
The `busyLooping()` would retry requested `onComplete()` signal for that specific error until success or timeout

(cherry picked from commit 7680e02)

spring-integration-core/src/main/java/org/springframework/integration/channel/FluxMessageChannel.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2015-2024 the original author or authors.
+ * Copyright 2015-2025 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.
@@ -86,15 +86,21 @@ private boolean tryEmitMessage(Message<?> message) {
 					.setHeader(IntegrationMessageHeaderAccessor.REACTOR_CONTEXT, contextView)
 					.build();
 		}
-		return switch (this.sink.tryEmitNext(messageToEmit)) {
-			case OK -> true;
-			case FAIL_NON_SERIALIZED, FAIL_OVERFLOW -> false;
-			case FAIL_ZERO_SUBSCRIBER ->
-					throw new IllegalStateException("The [" + this + "] doesn't have subscribers to accept messages");
-			case FAIL_TERMINATED, FAIL_CANCELLED ->
-					throw new IllegalStateException("Cannot emit messages into the cancelled or terminated sink: "
-							+ this.sink);
-		};
+
+		if (this.active) {
+			return switch (this.sink.tryEmitNext(messageToEmit)) {
+				case OK -> true;
+				case FAIL_NON_SERIALIZED, FAIL_OVERFLOW -> false;
+				case FAIL_ZERO_SUBSCRIBER ->
+						throw new IllegalStateException("The [" + this + "] doesn't have subscribers to accept messages");
+				case FAIL_TERMINATED, FAIL_CANCELLED ->
+						throw new IllegalStateException("Cannot emit messages into the cancelled or terminated sink: "
+								+ this.sink);
+			};
+		}
+		else {
+			return false;
+		}
 	}
 
 	@Override
@@ -171,7 +177,7 @@ private void sendReactiveMessage(Message<?> message) {
 	public void destroy() {
 		this.active = false;
 		this.upstreamSubscriptions.dispose();
-		this.sink.emitComplete(Sinks.EmitFailureHandler.FAIL_FAST);
+		this.sink.emitComplete(Sinks.EmitFailureHandler.busyLooping(Duration.ofSeconds(1)));
 		super.destroy();
 	}