Use RxJava 2/3 fromPublisher() when possible in ReactiveAdapterRegistry

Closes gh-26051

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

@@ -264,14 +264,12 @@ public class ReactiveAdapterRegistry {
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.multiValue(io.reactivex.Observable.class, io.reactivex.Observable::empty),
 					source -> ((io.reactivex.Observable<?>) source).toFlowable(io.reactivex.BackpressureStrategy.BUFFER),
-					source -> io.reactivex.Flowable.fromPublisher(source)
-							.toObservable()
+					io.reactivex.Observable::fromPublisher
 			);
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.singleRequiredValue(io.reactivex.Single.class),
 					source -> ((io.reactivex.Single<?>) source).toFlowable(),
-					source -> io.reactivex.Flowable.fromPublisher(source)
-							.toObservable().singleElement().toSingle()
+					io.reactivex.Single::fromPublisher
 			);
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.singleOptionalValue(io.reactivex.Maybe.class, io.reactivex.Maybe::empty),
@@ -282,8 +280,7 @@ public class ReactiveAdapterRegistry {
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.noValue(io.reactivex.Completable.class, io.reactivex.Completable::complete),
 					source -> ((io.reactivex.Completable) source).toFlowable(),
-					source -> io.reactivex.Flowable.fromPublisher(source)
-							.toObservable().ignoreElements()
+					io.reactivex.Completable::fromPublisher
 			);
 		}
 	}
@@ -304,30 +301,26 @@ public class ReactiveAdapterRegistry {
 							io.reactivex.rxjava3.core.Observable::empty),
 					source -> ((io.reactivex.rxjava3.core.Observable<?>) source).toFlowable(
 							io.reactivex.rxjava3.core.BackpressureStrategy.BUFFER),
-					source -> io.reactivex.rxjava3.core.Flowable.fromPublisher(source)
-							.toObservable()
+					io.reactivex.rxjava3.core.Observable::fromPublisher
 			);
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.singleRequiredValue(io.reactivex.rxjava3.core.Single.class),
 					source -> ((io.reactivex.rxjava3.core.Single<?>) source).toFlowable(),
-					source -> io.reactivex.rxjava3.core.Flowable.fromPublisher(source)
-							.toObservable().singleElement().toSingle()
+					io.reactivex.rxjava3.core.Single::fromPublisher
 			);
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.singleOptionalValue(
 							io.reactivex.rxjava3.core.Maybe.class,
 							io.reactivex.rxjava3.core.Maybe::empty),
 					source -> ((io.reactivex.rxjava3.core.Maybe<?>) source).toFlowable(),
-					source -> io.reactivex.rxjava3.core.Flowable.fromPublisher(source)
-							.toObservable().singleElement()
+					io.reactivex.rxjava3.core.Maybe::fromPublisher
 			);
 			registry.registerReactiveType(
 					ReactiveTypeDescriptor.noValue(
 							io.reactivex.rxjava3.core.Completable.class,
 							io.reactivex.rxjava3.core.Completable::complete),
 					source -> ((io.reactivex.rxjava3.core.Completable) source).toFlowable(),
-					source -> io.reactivex.rxjava3.core.Flowable.fromPublisher(source)
-							.toObservable().ignoreElements()
+					io.reactivex.rxjava3.core.Completable::fromPublisher
 			);
 		}
 	}