Change InvocableHandlerMethod#invokeSuspendingFunction return type

This commits changes the return type from Publisher<?> to
Object in order to avoid potential compatibility issues when
the Reactive Streams dependency is not in the classpath.

Closes gh-30716

spring-web/src/main/java/org/springframework/web/method/support/InvocableHandlerMethod.java

@@ -20,8 +20,6 @@
 import java.lang.reflect.Method;
 import java.util.Arrays;
 
-import org.reactivestreams.Publisher;
-
 import org.springframework.context.MessageSource;
 import org.springframework.core.CoroutinesUtils;
 import org.springframework.core.DefaultParameterNameDiscoverer;
@@ -240,7 +238,7 @@ else if (targetException instanceof Exception exception) {
 	 * instead.
 	 * @since 6.0
 	 */
-	protected Publisher<?> invokeSuspendingFunction(Method method, Object target, Object[] args) {
+	protected Object invokeSuspendingFunction(Method method, Object target, Object[] args) {
 		return CoroutinesUtils.invokeSuspendingFunction(method, target, args);
 	}