Refine Coroutines annotated controller support

This commit refines Coroutines annotated controller support
by considering Kotlin Unit as Java void and using the right
ReactiveAdapter to support all use cases, including suspending
functions that return Flow (usual when using APIs like WebClient).

It also fixes RSocket fire and forget handling and adds related tests
for that use case.

Closes gh-24057
Closes gh-23866

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

@@ -30,6 +30,7 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.function.Predicate;
 
+import kotlin.Unit;
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
 import kotlin.reflect.jvm.ReflectJvmMapping;
@@ -929,6 +930,9 @@ public class MethodParameter {
 				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
 				if (function != null && function.isSuspend()) {
 					Type paramType = ReflectJvmMapping.getJavaType(function.getReturnType());
+					if (paramType == Unit.class) {
+						paramType = void.class;
+					}
 					return ResolvableType.forType(paramType).resolve(method.getReturnType());
 				}
 			}