Defensively register Flow bridge in delegate class

See gh-31000

spring-core/src/main/java/org/springframework/core/ReactiveAdapterRegistry.java

@@ -86,7 +86,6 @@ public class ReactiveAdapterRegistry {
 	 * Create a registry and auto-register default adapters.
 	 * @see #getSharedInstance()
 	 */
-	@SuppressWarnings("unchecked")
 	public ReactiveAdapterRegistry() {
 		// Defensive guard for the Reactive Streams API itself
 		if (!reactiveStreamsPresent) {
@@ -115,10 +114,7 @@ public ReactiveAdapterRegistry() {
 
 		// Simple Flow.Publisher bridge if Reactor is not present
 		if (!reactorPresent) {
-			this.adapters.add(new ReactiveAdapter(
-					ReactiveTypeDescriptor.multiValue(Flow.Publisher.class, () -> PublisherToRS.EMPTY_FLOW),
-					source -> new PublisherToRS<>((Flow.Publisher<Object>) source),
-					source -> new PublisherToFlow<>((Publisher<Object>) source)));
+			new FlowBridgeRegistrar().registerAdapter(this);
 		}
 	}
 
@@ -375,6 +371,18 @@ void registerAdapters(ReactiveAdapterRegistry registry) {
 	}
 
 
+	private static class FlowBridgeRegistrar {
+
+		@SuppressWarnings("unchecked")
+		void registerAdapter(ReactiveAdapterRegistry registry) {
+			registry.registerReactiveType(
+					ReactiveTypeDescriptor.multiValue(Flow.Publisher.class, () -> PublisherToRS.EMPTY_FLOW),
+					source -> new PublisherToRS<>((Flow.Publisher<Object>) source),
+					source -> new PublisherToFlow<>((Publisher<Object>) source));
+		}
+	}
+
+
 	private static class PublisherToFlow<T> implements Flow.Publisher<T> {
 
 		private static final Flow.Subscription EMPTY_SUBSCRIPTION = new Flow.Subscription() {