add test for StatelessSession.upsert()

hibernate-core/src/test/java/org/hibernate/orm/test/stateless/StatelessSessionTest.java

@@ -199,7 +199,7 @@ public class StatelessSessionTest {
 
 		scope.inStatelessTransaction(
 				statelessSession -> {
-					Paper p2 = (Paper) statelessSession.get( Paper.class, paper.getId() );
+					Paper p2 = statelessSession.get( Paper.class, paper.getId() );
 					p2.setColor( "White" );
 					statelessSession.update( p2 );
 				}

hibernate-core/src/test/java/org/hibernate/orm/test/stateless/UpsertTest.java

@@ -0,0 +1,45 @@
+package org.hibernate.orm.test.stateless;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Id;
+import org.hibernate.testing.orm.junit.DomainModel;
+import org.hibernate.testing.orm.junit.SessionFactory;
+import org.hibernate.testing.orm.junit.SessionFactoryScope;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.Assert.assertEquals;
+
+@SessionFactory
+@DomainModel(annotatedClasses = UpsertTest.Record.class)
+public class UpsertTest {
+    @Test void test(SessionFactoryScope scope) {
+        scope.inStatelessTransaction(s-> {
+            s.upsert(new Record(123L,"hello earth"));
+            s.upsert(new Record(456L,"hello mars"));
+        });
+        scope.inStatelessTransaction(s-> {
+            assertEquals("hello earth",s.get(Record.class,123L).message);
+            assertEquals("hello mars",s.get(Record.class,456L).message);
+        });
+        scope.inStatelessTransaction(s-> {
+            s.upsert(new Record(123L,"goodbye earth"));
+        });
+        scope.inStatelessTransaction(s-> {
+            assertEquals("goodbye earth",s.get(Record.class,123L).message);
+            assertEquals("hello mars",s.get(Record.class,456L).message);
+        });
+    }
+    @Entity
+    static class Record {
+        @Id Long id;
+        String message;
+
+        Record(Long id, String message) {
+            this.id = id;
+            this.message = message;
+        }
+
+        Record() {
+        }
+    }
+}